Use an internal window (instead of a real OS window) for popup choices,
and cap the maximum size. Use a scrollbar when the number of items is
too large, and adjust the position to fit in the window.
Use The Qt 5.6 Shortcut item, but via a conditional file so
we don’t touch the base Qt requirement. (This means no shortcuts
when using Qt 5.4 or 5.5, oh well)
This simplifies the launcher's rendering since the widget-based
code is gone, various things get hooked up as a result. Styling
fixes everywhere as well.
Menubar on Linux/Windows needs to be re-added.
