Add a proper menu to make functionality more discoverable

[hoechenberger]

In #79 it became apparent that not even very sophisticated and knowledgeable users like @agramfort are not aware of all the cool functionality baked into this tool. This is partially because these features are difficult to discover and only usable via keyboard shortcuts, for which we provide a rather poor table after clicking the Help button.

I propose we add a proper menu that makes for easier discoverability of both the functionality and their associated keyboard shortcuts (as they appear next to the menu entries on most (all?) systems).

[hasrat17]

Hey @hoechenberger, I think this issue is already resolved. Is anything left in this? As now, in help, it is listing all the keyboard shortcuts.

[hoechenberger]

Hello, this is not a menu...

I started working on one in #126