Plasma::Theme::wallpaperPath(const QSize&)

a nice improvement that fixes a small handful of packaging bugs and nuisances.
it allows us to do away with harcoding the wallpaper in multiple places (previously 3 places) in application code. as an extra bonus is also allows:

* the theme to define the default wallpaper
* the theme to ship with a default wallpaper (perhaps later on we can load wallpapers shipped with a theme in the config dialog?)
* the default wallpaper to defined in the plasmarc file (so downstream doesn't have to hack the code to change it)