StyleItem: Resolve item palette per item type

This queries the platform theme for each type, and sets the right
palette. This should allow us to remove some workarounds we might
have for those cases in desktop style.

Change-Id: Ia5f650b46de5418586bc946d63fd394875e90184
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>