IGV: make "Selection mode" a toggle button

In IGV, an open graph has two modes for interaction:
- "Panning mode": the user can move the graph with the mouse.
- "Selection mode": the user can drag a selection rectangle with the mouse to select multiple nodes. This mode can also be accessed by holding down the CMD/Ctrl key on the keyboard.

All modes have a toolbar button to activate them. And exactly one of the two modes is always selected.

To make the toolbar clearer (and easier), remove the "Panning Mode" button and make the "Select Mode" button a toggle button: When it is selected, the user is in "Selection Mode", otherwise in "Panning Mode". The default mode would be "Panning Mode".