Add L/R channel routing for VST plugins

[messmerd]

Mono VST plugins in LMMS are not handled correctly. They only output audio in the left channel rather than upmixing their mono output to stereo. (See #6558)

I wanted to solve this problem but also allow more flexibility in how mono audio is handled, such as using one channel while bypassing the other. To do this, I've added a PluginPortConfig class which allows users to select a method of routing LMMS's stereo audio in and out of a mono-in, mono-out, or mono-in/out plugin. This is another feature from #7199 which I am moving to its own PR.

With PluginPortConfig, all mono VSTs have an automatable combo box in their parameters window to allow user control over how the mono plugin is integrated into the stereo DAW. The design is generic and will allow LV2 and possibly LADSPA to use it in the future.

By default, audio is down-mixed to mono for mono-input plugins and up-mixed to stereo for mono-output plugins.
There is also the option to only use one channel for input/output and bypass the other. This will be useful for mono effects, since users will be able to chain two effect instances together to create what is essentially a true stereo plugin if they are unsatisfied with up-mixing mono to stereo.

I also removed the "Close" button widget from the VST parameter windows since they are useless and were making it difficult for me to add the port configuration combo boxes.

Here's a chart of all possible routing configurations for each plugin type:

TODO:

• Rename PluginPortConfig or keep the name?
• Test with other mono VSTs - especially effect VSTs
• Test loading and saving configurations
• Add images to the ComboBox to visualize each configuration?
• Add a helper class for routing audio based on the current configuration?
• Audio bypass?
• More stereo routing options?

Fixes #6558

[JohannesLorenz]

It should be noted (also in the headline of this PR) that this only affects RemotePlugin based mono plugins (at least, LADSPA and Lv2 are currently not affected).

[JohannesLorenz]

It should be noted (also in the headline of this PR) that this only affects RemotePlugin based mono plugins

Well, if you plan to do it for all, the headline is correct, though 😃

[messmerd]

Marking this as a draft because there's some refactoring I want to do, and a couple minor features from the TODO list that I want to try adding.

I expect to complete this work within the next few days.