Hiding tab bars

[LukeP0WERS]

This adds the option to hide/show tab bars for a given leaf, see #321.

• Adds hidable_tab_bars option to DockArea that enables tab bars to be hidden per-leaf.
• When hidden, the tab body's top inner margin acts as a drag handle for the active tab, with the same grab/grabbing cursor behavior as regular tabs.
• A configurable triangle button in the top-left corner allows showing the tab bar again.
• Right-clicking the top margin also provides a "Show tab bar" context menu.
• When a tab is dragged out of a leaf with a hidden tab bar, the tab bar automatically unhides so remaining tabs stay accessible.

TODO

• Consider adding an example to demo the new feature.
• Consider whether a setting to disable dragging via the top margin would be needed.

Note:
I think there should be an interface to display a custom button in place of the triangle, but that would better to include in a PR later on that adds custom button UI for all the buttons.