Various improvements to textbox input

[smoogipoo]

• Fixes TextBoxes blocking all key input.
  • Not entirely sure why this was here, if any case comes up where this breaks we'll analyse it/comment it appropriately.
• Removes unnecessary returns which are already handled in OnAction.
• Fixes PlatformActions not being handled before keys.
  • Fixes not being able to Cmd+Left/Cmd+Right in the search box in SongSelect.
• Fixes readonly textboxes being able to be tabbed to. Browsers do the same thing.

[peppy]

Breaks song select left/right arrows.

[peppy]

Correction: breaks left/right arrow keys game-wide (slider bars don't work, for instance). I think this is related to the priority change.

[UselessToucan]

I think you should merge #1365 and #1362 before working on this PR because it will allow you to see how everything fits together.

[peppy]

We generally merge required fixes before breaking changes.

[smoogipoo]

The reason why I did this is to fix the behaviour in those PRs prior to merging them. They're my target but there are a few interactions and quirks with input handling before those can be merged.

Focus is one last thing remaining before those two can be merged.