Drop empty text="" attribute from xml files

Should help reduce bloat, and this is the default anyways.
Drop empty helptext="" attribute from xml file
Spaces to tabs

Most of our xml files indent with tabs, so let's do in these files too.
Drop border_size="0" attribute from xml file

0 is the default and never changed anywhere.
Make vertical_scrollbar=1 the default

If there is something to scroll to, it should be reachable.
Drop horizontal_scrollbar="0" attribute from xml files

The default and never overwritten in styles.
Drop background_color="255,255,255,0" attribute from xml files

No ScrollArea should be invisible without indication of scroll-ability.
This causes all modified scroll areas to take on the default scroll area
styling. Our trade tab was slightly modified to accomodate the changes.
Remove commented xml markup

Not likely to be used anytime soon.
Remove commented and obsolete health display code in gui

We can use HealthWidget for anything worth displaying it in.
Merge pull request #2004 from unknown-horizons/empty-xml-attributes

Mostly gets rid of useless or redundant attribute definitions in xml files,
taking up space for no gain and complicating our gui code even further for
anybody new to it.

The ScrollArea behavior change should also help users in that they now
recognize when scrolling is possible and attach this to seeing the slightly
shaded background.