Fix Color Selector popup and color preview auto-hiding

There were two problems:

1. There is a bug either in Qt or in X11: if one moves

mouse cursor from a normal window to its tooltip, this
normal window never gets a QEvent::Leave event. Even if
the user switches the task using Alt+Tab. This is
worked-around by tracking Enter/Leave events in the tooltip
itself. Good that we have access to it :)

2. When using Qt::X11BypassWindowManagerHint the popup window

gets no sane events from the window manager on X11, therefore
we cannot hide that popup. The problem is solved by making
the window just a normal window.

BACKPORT:krita/4.1
(cherry picked from commit fa8f98c5286823e2bea0b6990a01e356850a9bc5)