progressbar: Fix setting background image in corner cases

In particular with a widget that would be loaded without setting its xml
arguments properly (currently only aware of the loading screen). Employs
the same hackish and questionable technique used for the fill image.
Fix TilingProgressBar passing floats where ints were expected
Loading screen: Drop unnecessary size and wrap_text attributes

wrap_text defaults to 0 and thus the sizes weren't doing much here in
any case while limiting the text width might harm translations still.