Add right-click menu with tree properties / other options

[fedarko]

Something I just thought of -- when you right click on an Emperor plot, you get a nice menu that lets you perform some common operations:

But when you right click on the canvas in Empress, nothing happens (well, nothing useful, anyway):

It'd make for a more pleasant user experience if Empress had a "right click menu" similar to Emperor's. We could add in some of the tree properties options to this menu (as well as SVG exporting when #218 is merged in). I think the added consistency here would be really nice, as would the ability to quickly recenter the camera in Empress.