progressbar: `fill` attribute for xml definition

Also roll back ProgressBar to one resizable image instead of tiling:
* There were too many images with a TilingBG of 1px tiles
* Stretching 1px tiles via pychan yields the exact same result as tiling

Might add a TilingProgressBar sometime later if we decide it's useful.
TilingProgressBar

What a mess. Refs #1650.
Currently unused but i guess we can find use cases for this and it's not
completely unmaintainable either. Will count size of left and right bar
frames towards the progress displayed (i.e. the max_size).
Usage:
<TilingProgressBar background="bg.png"
left="left.png" fill="fill.png" right="right.png" />