consolidate pixmaps

[illwieckz]

Currently there is two folders for pixmaps:

pixmaps/
src/xpm/

Pixmaps in src/xpm are source files compiled within xqf binary, so it makes sense to have them in src/ folder, but nothing prevent us to do something else for them, or for pixmaps in pixmaps/.

Also, the xqf.png in multiple size (32, 48), and other icons can be generated at make time from xqf.svg. So, it's a source, too.

Since the splash screen is no longer displayed (see #93 and 874c92a), we can drop the splash image too.

[jmallach]

I have a patch that implements parts of the comments you make. Just needs some testing. I'm not decided if I personally care about generating png from svg, instead of ensuring we ship the source for anyone interested.

[illwieckz]

We need to re-add flags since they are lost (see #147), so we will be able to fix this issue.