Portable Warzone fixes for windows.
Fix windows paths.

Windows portable icon by forum member Duda.
Portable header & welcome images from forum member Aubergine.
Image fixes supplied by Berg.

Note: In configure stage (Windows only!), you can specify --enable-portable [default no] to
      create a portable installer. (That is, everything is self-contained in the directory
      you install Warzone 2100 to.  It will *not* use any other directory on the system.)
      If you pass --with-videos [default no], then it will add the videos with the portable
      installer and looks for the video file in \opt\sequences.wz

FIXME: Need: RUSSIAN, GERMAN, DUTCH, tranlations fixes for the warzone2100.nsi file.
(cherry picked from commit c1b29c1f5e6e6331ab5a96f27ba963192e709d81)

Conflicts:
	src/Makefile.am
Add filter icon by duha
Add refresh icon by slate

Fix button states.