#1 Support for Dockable maximize/restore

[Nils Meier]

Use Case
- Dockable has the focus and user presses Ctrl-M or clicks on (optional) Toolbar button "Maximize"
- Current dockable takes up all available space in the DockingPane
- User presses Ctrl-m a 2nd time or clicks on the (now showing) "Restore" button in the Toolbar
- Dockable returns back to the old size (all other previously visible Dockables appear as before)

Extensions
- Putting a new Dockable into the pane should restore the currently maximized Dockable
- Removing a currently maximized Dockable should restore to a view without any maximized Dockables
- The state of being maximized should be remembered for that Dockable on restore of the pane state