Implement "Focus Places Panel"

This commit implements an action to move focus to the Places panel analogous to how "Focus Terminal Panel" is implemented. While doing so I noticed some room for improvement code-wise. Those are separated into a different commit but kept here because it makes sense semantically to look at both "Focus Panel" actions at the same time.

The "Focus Terminal Panel" action is moved into the "Show Panels" submenu 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.

I changed it so 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.