Panels

Goals

We want a new panel system that allows for more flexibility in Builder.

A new hover layer, that allows panels to be hidden, and float over the content area when necessary. They are transient, and when focus leaves, they should hide themselves after a small timeout.
- Pressing Alt should animate in the panels enough to read their titles (which will likely look like "tabs" for the floating layer).
- Pressing double Alt should fully animate in the floating panels
- These should be focusable with keyboard shortcuts
It should be possible to split the panels, so we can have two panels stack on top of each other.
- There are really two modes of "stacking" here.
  - A true split, like a GtkPaned, where both are always visible
  - The splits are more like a GtkBox inside a Viewport/ScrolledWindow, and you scroll them as a set
Tearing out a panel should be possible, and things like ide_widget_get_workbench() should still work.