Add "select all" option

[fedarko]

To just select all nodes / edges / patterns in the graph. It'd be really neat to map this to ctrl-A (or command-A for mac users), although we should probs also add a UI element somewhere that does this by clicking.

Maybe also add an "unselect all" option? We could then include both of these in the "Selected Elements" section of the menu, or something. This would pair nicely with #224, since it would enable the user to do stuff like select all nodes and then immediately see "oh huh, so that's the average node length".