Add focus follows mouse option for widgets

Some applications have multiple panes in one window, each of which may require keyboard focus at a given time. (E.g. A windows with two or more main textareas.) On linux, many window managers (kwin, i3, etc.) allow for a "focus follows mouse" policy for windows, which sets window/keyboard focus to whichever window is under the mouse. It would be more consistent to enable this policy for multi-pane applications.

One way of doing this would be to add a new option to Qt:FocusPolicy (e.g. Qt::(Mouse/Hover)Focus) that delivers keyboard focus when the mouse enters the area over the target widget (tab would naturally switch further along the focus chain). Such events would fall under Qt::MouseFocusReason.

This new option would not break existing systems, and, since focus-follows-mouse behavior is typically only present on Linux computers, need not apply to all platforms.

(This request is inspired by https://bugs.kde.org/show_bug.cgi?id=64816.)