Add RectF editor, improve grid display, and support translated/untranslated values

Summary:

• extensively use current QLocale for displaying values; use C locale for internal representation of values
  • separate translated from non-translated API for obtaining value strings
• improve indentations and borders of combo, cursor, linestyle editors for various QStyles
• assume that invalid grid color means "no grid" for the editor view
• improve display of editors for when editor view's grid is off
• redraw the view when changing grid settings
• build color combo delegate even if KPROPERTY_KF is OFF
• add RectF editor
• temporarily use int spinbox editor for (u)longlong and uint types
• linestyle combo: improve drawing line previews
• font editor: properly use the current font when showing the font dialog
• kpropertyexample app:
  • add test for precision of floating-point numbers
  • fix font face
  • add "Show grid" and "Show frame" check boxes for testing changes of grid and frame settings

Test Plan:

• run the kpropertyexample app using various QStyles, test all editors
• look at the new APIs

Reviewers: piggz

Subscribers: Kexi-Devel-list

Differential Revision: https://phabricator.kde.org/D665