Keep tab bar shown (expanded) if one of triggers is still there. (#552)

The tab bar can be shown by three reasons: mouseover, keyboard shortcut,
and feedback for new tabs. However, sometimes two or more triggers can
be fired in a same time. Now TST keeps the tab bar showing (expanded)
for combinated cases, like: shown by the keyboard shortcut, the mouse
goes onto the tab bar, and it leaves from the tab bar.