Add shortcut to toggle profile switcher

Press 'G' to open the profile switcher, select a profile with the arrow
keys, apply with 'Enter' and close the switcher with 'Escape'.

While 'T' and 'P' will be / were already taken, 'G' is at least easily
reachable with the left hand for users using a mouse with their right
hand at the same time.

Since Bootstrap keeps updating the tooltip when changing options in the
dropdown (which is useful in case the text is longer than the width of
the control), the shortcut text needs to be applied dynamically too.

locales/en.json

@@ -171,7 +171,8 @@
       "trekking-steep": "Trekking bike (steep)",
       "vm-forum-liegerad-schnell": "Recumbent bike (fast)",
       "vm-forum-velomobil-schnell": "Velomobile (fast)"
-    }
+    },
+    "profile-tooltip": "\n(G key to switch)"
   },
   "sidebar": {
     "analysis": {