Improve entry mask

Improve and simplify the way masks work on entries.

* Remove the idea of fields. It's not something useful and no other mask
  system out there uses something like it. Now the entry will behave like the
  mask is not really "text" until it's really filled.
* Remove the TODOs. Optional chars are only possible if we insert static chars
  on demand, which is not the case here.
* Fix and reenable some tests. Instead of not running them, skip on win32

Change-Id: Ic59ea29dbd4ac86e59b40d58ef8ec89962c6ac4a