Normalize strokes in fixed order (#1572)

* Use normalized strokes when removing duplicates only when appropriate

* Use normalized stroke in `keymap.unbind`

* Normalize strokes by sorting the modifiers before the keys

This also sorts the modifiers in a fixed manner, decided by 
`modkeys.keys`.
We need to do this because we display the strokes in a few places like 
the command palette.

data/core/modkeys-generic.lua

@@ -9,6 +9,6 @@ modkeys.map = {
   ["right alt"]   = "altgr",
 }
 
-modkeys.keys = { "ctrl", "alt", "altgr", "shift" }
+modkeys.keys = { "ctrl", "shift", "alt", "altgr" }
 
 return modkeys