Show a splashscreen while loading Krita.

The splash image is included from an xpm file since we cannot
use the application data directories before having setup all
the standard directories, so we cannot dynamically load a file.

Note that gimp and krita create xpm's that give lots of compile
warnings, so after changing the splash image, we need to add
a bunch of consts:

turn

static char * splash_screen_xpm[]={

ito

static const char * const splash_screen_xpm[]={