Bind the mouse to the scan methods.

Now you can scroll the star map by "grabbing" the background with the
left mouse button and dragging.  Super-cool that Tk(inter) makes this so
simple.

ui.py

@@ -69,7 +69,13 @@ class App:
         scrollY.grid(row=0, column=1, sticky=N+S)
         scrollX.grid(row=1, column=0, sticky=E+W)
 
-        canvas.bind("<Configure>", self.handle_canvas_resize)
+        scrollX['troughcolor'] = scrollY['troughcolor'] = DARK_GRAY
+        scrollX['bg'] = scrollY['bg'] = '#ddf'
+
+        canvas.bind('<Configure>', self.handle_canvas_resize)
+
+        canvas.bind('<Button-1>', lambda event: canvas.scan_mark(event.x, event.y))
+        canvas.bind('<B1-Motion>', lambda event: canvas.scan_dragto(event.x, event.y, gain=7))
 
         # Star System