Folding Panels

While the UI panels will be resizable, sometimes you'll just want to get one out of the way completely for a bit, and sometimes it'd be nice if you could devote all the window real estate to a single panel for a while. Eclipse does this; each panel has a minimize and a maximize button (except the central editor panel, which can only be maximized). I think we should follow Eclipse's lead here, at least a little bit.

• Minimizing:
  The panel is reduced to nothing but its tabs at the edge of the window. Clicking any one of the tabs restores the panel to its prior state with that tab as the current one.
  
• Maximizing:
  The panel is expanded to fill the entire window while the other panels are hidden; the minimize button is still available, and the maximize button takes on a depressed appearance (it is a toggle, after all). Maximized panels may change appearance slightly and take on a few features of other panels which are hidden.

The maximized behavior for each panel is as follows:

1. Assets
   The asset panel becomes multi-column. Double-clicking on an asset to open it in the editor pane un-maximizes the asset panel to reveal the editor. Assets that open in external programs should probably not un-maximize it if opened.
2. Editor
   The editor gains a widget at the bottom which has the playback cursor portion of the timeline.
3. Properties
   The properties panel gains a viewer widget showing the asset in the editor window as well as a widget with the contents of the parameter curve editor tab from the timeline panel.
4. Timeline
   The timeline panel gains a view widget, and also the parameters part of the properties panel.

This is a little different to Eclipse's behavior, but I hope more useful.