Implemented user editable date acceptance patterns.

The introduction of strict date parsing using locale dependent date
acceptance patterns in 3.6.0 wasn't always welcomed. Besides that not
every locale had patterns for incomplete (only day and month) date
input, users also complained about not being able to key in dates on
numeric keypads if the locale's date separator wasn't '/' or '-'

This commit implements a "Date acceptance patterns" edit field under
Tools->Options->LanguageSettings->Languages that follows the selected
locale and enables the user to add patterns.

Example de-DE locale:
* default patterns: D.M.Y;D.M.
* to enable additional input on numeric keypad: D.M.Y;D.M.;D-M-Y;D-M
  * if 3-4 shall not result in a date, D-M- could be used instead of D-M
  * note that to enter an ISO 8601 Y-M-D date with a D-M-Y pattern
    active one needs to enter a year >31 or with at least 3 digits, e.g.
    011

Change-Id: I9e20fcb168578b02d0f0248a0e209942e0f27113