Rather than control the propagation of the value between the slider and
the textbox, add a property that controls the propagation of the value
between the bindables inside the form control to external bindables.
This will help alleviate issues where the external bindable update
incurs overheads due to having heavy change callbacks attached.
RFC. As per
https://discord.com/channels/188630481301012481/188630652340404224/1291346164976980009.
The diff is extremely dumb but I tried a few smarter methods and they're
either not fully correct or don't work. Primary problem is that menu
items are mutable externally and there's no hook provided by the
framework to know that items changed. One could probably be made but I'd
prefer that this change be examined visually first before I engage in
too much ceremony and start changing framework around.
These aren't used in many places, but we've since moved away from
opacity in UI elements like this, so let's just nuke it here for
legibility.
Addresses https://github.com/ppy/osu/discussions/29906.