Graphics update for progressbar
Background image for progress bar. Fixes #1650

And somewhat minor refactoring. Now exposes attributes to xml.
Changed behavior which can be extended/generalized later:
* Fixed background size of currently 100px; has to match max amount
* Fixed progress tile of currently 1px
* No special left/right framing tiles
Use background attribute for ProgressBar
Call tile-laying even if no background exists

Turns out it is not important to draw the background behind other stuff.
int() for progressbar amount

Boat builder for instance was passing floats here.
"name" instead of 'name' for xml file

In the name of coherence.
Boat builder layout update

The funny max_size there restricts the bar height and thus scales all
icons, which is easier interface-wise than implementing own "height"
attributes and also more flexible even if weird-looking at first.
Merge pull request #2007 from unknown-horizons/progressbar

Add background feature to progress bar and relayout it

This is a first iteration but should still work as-is.
In particular it'd be nice to have a horizontal resize feature instead
of hardcoding the exactly 100 tiles as widget size.
Bar height can be worked around using a max_size height component while
not restricting width.