Feathers theme aware fixes

[gagnus]

Objective

• Developing a light theme I noticed a few bugs:
  • slider didn't recolor until mouse over
  • slider behind number input (when SoftLimit is used) had same issue
  • icons, including dialog close never recolor, made them take text color from theme
  • disclosure toggle recolored only when interacted with

Solution

• Each had a slightly different fix. Disclosure toggle was setting colors directly when enabled/disabled, instead re-inserted InheritableThemeTextColor.
• NumberInput needed a plugin to add an update function
• Slider needed an update function
• icons I added a new ThemedIcon component marker and a system to color that after text color has propagated (runs after propagation)

Testing

• Tested all by switching theme in a local app. Should be zero change if you are not changing theme as bugs only show up when doing so.

[viridia]

Note this will have a merge conflict with #24701

[viridia]

Aw darn, I was hoping I could do the number input purely with observers - I wanted to see if it was possible. No such luck I guess :(

[gagnus]

yeah I was hunting around looking for the plugin! :)