Keep track of last language layout used when using input-method

This allows us to switch back when e.g. switching away from the terminal layout.

This also adds some debugging aid as I've seen the OSK not unfold with earlier versions of this code. Hence marking as draft.

* doc: Be a bit more specific which entry to select

* input-surface: Make it simple to check for language layouts

  This makes it simple to distinguish "language" layouts from
  "special" layouts like emoji or terminal.
  
* input-surface: Make it simple to check for terminal layouts

  This makes it simple to distinguish the terminal layout from
  language or other layouts.
  
* input-surface: Ignore "special" layouts when remembering last layout

  This allows us to switch back to the last language layout. The only
  special layout is currently the terminal layout but there will be more
  like PIN.
  
* input-surface: Use widget height

  The configured height doesn't really matter
  
* input-surface: Add callback that aborts if animation didn't finish

  I've seen the keyboard not unfold. Let's have some debugging help.
  

Helps: #14 (closed)