Implement "Focus Places Panel"

Implement "Focus Places Panel"

This commit implements an action to move focus to the Places panel
analogous to "Focus Terminal Panel" functionality-wise.
The implementation of the "Focus Terminal Panel" and "Focus Places
Panel" actions is streamlined while improving their code quality.

The "Focus Terminal Panel" action is moved into the "Show Panels"
sub-menu because it makes more sense to be there considering that its
previous location (the "Tools" menu) is meant for external applications
and not for functionality internal to Dolphin.

This commit also makes it so the keyboard focus is moved to and from
the Places panel whenever it is toggled visible or invisible. This is
now consistent with the focus handling when the Terminal panel is shown
or hidden.

The "Focus Places Panel" is one of the actions which was wished for in
KDE's accessibility chat room because people relying on keyboard
controls might need to press the Tab key a lot to move from the view to
the Places panel.

The new default shortcut is Ctrl+P.