tree: Handle popup menu shortcuts (Menu key and Shift+F10)
Currently it isn't possible to open the context menu of the project tree so it's hard to do anything useful with the project tree with keyboard only.
So this commit now makes the IdeTree handle those keys, by connecting to the GtkWidget's popup-menu signal.
Seems like the context menu is shown underneath instead of on the side, mostly because I didn't move out the code that does it in a common place for both the _popup() function and the popover-menu signal one. Should I make them behave the same way?