The parent owner of the TabBar changes as it is added to layout objects
within other widgets. So the parent we set at the beginning is no longer
the parent when we right-click on a tab. We now just store the TabWidget
directly.
QTabWidget's built-in tab bar management made it difficult to properly
position the new tab button, and caused visual artifacts with the
separator line across themes.
Replace QTabWidget with a custom TabWidget that places a QTabBar
alongside an explicit new tab button. This us gives full control over
the tab bar row layout.
