#92 "Swap Panel" and "Synchronize Panel" only work with file tab

[Jörn Zaefferer]

"Swap Panel" and "Synchronize Panel" items in the file
menu do nothing when the user selecteds a panel other
then a file panel (eg. "Find in Files"). In that case,
they need to be either deactivated or removed.

Actions defined in ActionsSet can have "selection" and
"enablement" elements. Maybe those can be used to
control the enablement of the action.