[kwin] Use a QIcon in Client for the icons instead of Pixmaps

Client used to have dedicated methods for different icon sizes instead
of combining all pixmaps into one QIcon. This resulted in various parts
of KWin having different access to the icons:
* effects only got one pixmap of size 32x32
* decorations only got the 16x16 and 32x32 pixmaps combined into a QIcon
* tabbox could request all icon sizes, but only as pixmap

Now all sizes are available in one QIcon allowing to easily access the
best fitting icon in a given UI.