is_button2_event() was used to detect the user's desire for a
momentary-click on some buttons (mute, solo, mixer scenes)
is_momentary_push_event() disambiguates this action from is_button2_event()
for the special case of momentary, we can drop the workarounds for
the lack of middle-mouse buttons on Mac, and instead just use
shift+left-click on all platforms.
12e703414d