UI: Add frontend event and output signal for replay buffer saving

[Penwy]

Description

This adds a frontend event and a signal on the replay buffer's signal
handler that trigger when the replay buffer is saving (i.e. when a save
has been requested, but before it saves).
The frontend event triggers the moment the frontend requests a save
(either through hotkey or the button), whereas the signal triggers when
the replay buffer actually starts saving (separated by the encoding delay).

N.B.: Allowing the save hotkey to trigger a frontend event required
moving its registration from obs-ffmpeg (in the definition of the replay
buffer output) to the UI (with all the frontend hotkeys for outputs).
This actually constitutes the biggest change in this PR.

Motivation and Context

Motivation is mainly to bring replay buffer save to feature parity with
other output events, which almost all have both "before" and "after" events.
A specific usecase envisionned was to allow the dynamic changing of
replay buffer saving path/filename rather than having one set-in-stone
save formatting for the whole session, but many other usecases could be
possible.

Moving over the hotkey registration also has the added benefit of moving
its location next to the other replay buffer hotkeys in the settings UI,
which creates a more consistent UX.

How Has This Been Tested?

This has been tested on Ubuntu 22.04.
Changes to hotkey registration seem to not have affected its behaviour.
Event and signal trigger correctly from hotkey and button.

Types of changes

• New feature (non-breaking change which adds functionality)

• Documentation (a change to documentation pages)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

Is the hotkey member of struct ffmpeg_muxer used anywhere at all now? Should it be removed?

[Penwy]

the hotkey member isn't used anywhere in obs code as far as I can see, but the commit that put it there, 87a64e6 , states it does so to "Allows the ability to use it for more output types". So I preferred to not remove it in case it was used by some third party's output.
If you think that's mistaken, I'll gladly remove it.

[gxalpha]

Are the changes here all dependent on each other? If not, it might make sense to split them into multiple PRs.
It might also make sense to combine "UI: Register the replay buffer save hotkey in the frontend" and "obs-ffmpeg: Clean up now-redundant hotkey registering code" into one commit, given that otherwise you’d have two replay buffer hotkeys at the same time if you only have one of the commits.

Would also like a @Warchamp7 review on the fact that the save replay buffer hotkey is now always available, even if replay buffer is disabled.

[Penwy]

The two parts ("add signal and event" and "move hotkey registration") are not technically dependent on each other.

However, if the hotkey isn't moved, saving by pressing the hotkey will not trigger the firing of the frontend event (unless you can access the frontend api from within ffmpeg-mux). Event will only be sent when pressing the UI button, which does not feel to me like wishable behaviour.

[Warchamp7]

Would also like a @Warchamp7 review on the fact that the save replay buffer hotkey is now always available, even if replay buffer is disabled.

I am okay with this change

[Penwy]

(from what I can see the failed checks are just a result of temporary changes to the CI when this was last pushed, not of the changes introduced by this PR)

[NullFragment]

I just wanted to selfishly poke over here and see what's up. It'd be nice to have this functionality for using plugins with the replay buffer in the main branch instead of having to build with these changes pulled or use custom build artifacts.

@gxalpha is the only concern with this change right now (other than merge conflicts) that the two commits for moving the hotkey should be squashed into one?