[Gtk] rewrite DatePicker backend
[Gtk] rewrite DatePicker backend

 * fix component selection
 * fix (keep) seconds when other component chages
 * fix keyboard input (incl. arrow keys and separators)
 * fix mouse selection
 * add internationalization support
   * use localized separators
   * use localized component order
   * no 12/24 support
 * add DatePickerStyle support (show only relevant components)
 * add clipboard support

hamekoz/xwt,antmicro/xwt,iainx/xwt,residuum/xwt,mono/xwt,lytico/xwt,steffenWi/xwt,hwthomas/xwt,TheBrainTech/xwt,cra0zy/xwt,akrisiun/xwt