With this patch the Qt Quick Designer supports:

*Flat
*Flat Dark
*Flat Light
*Classic

I did not test the dark style.

I added a template engine for the .css files. The syntax for the .css
files
is identical to the QML syntax.

Known issues:
* The icons in the property editor do not play well with the
  Flat Light style. We have to get proper Core:Icon support for the
  icons
  and most icons have to be simplified/flattened to fit the new style/
  theme.
* Some colors in the connection editor are still dark and hardcoded. But
  this looks ok together with all themes.
* The highlight colors are not taken from the theme, yet.

Change-Id: I94f2b75f92db8e131c2b15867dbabe9ae4b2f25a
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>