I don't know why the viewports are deleted with deleteLater(), but it leads
to an OpenGL context getting deselected behind our back, and so we don't know
when is a good time to re-select it. This leads to termination of the engine
when selecting File->Close (Ctrl+W) in the editor, because the graphics
re-initialization fails with no GL context active.
Instead, just delete the viewport widget immediately, which works fine at
least on Linux. This is also recommended by the Qt documentation at
http://doc.qt.io/qt-5/qopenglwidget.html.
The custom title bar caused a lot of problems. Particularly turning it on and off between docked and undocked modes caused some internal trouble with the rendering going off. It also didn't have a close button.
Fortunately, the desired font and highlight effect of selected viewports can also be achieved with style sheets on the dock.
The SDL port used to define a separate set of key codes differing from
the usual ones. The SDL codes are now translated to make the Qt input
easy.
This also fixes a broken keycode for the '0' key.