VCL: Reorganize and rename wallpaper functions

Grouped wallpaper functions in outdev.hxx, also changed the function
names:

+ ImplDrawColorWallpaper  -> DrawColorWallpaper
+ ImplDrawBitmapWallpaper -> DrawBitmapWallpaper
+ ImplDrawWallpaper       -> DrawWallpaper

Change-Id: I0eb9c21bf2f6a336df22cd27cd1557f2c795f792