Various modifications to ppsspp-qt-git PKGBUILD

-Fix new path thanks to the change of PPSSPP executable name from this
commit:
https://github.com/hrydgard/ppsspp/commit/404766ce335e6ccd7441d8451b2a1beef91a54a4
-Add manpage.
-Add icons for various resolutions based on new PPSSPP assets/unix-icons
folder.
-Remove custom .desktop file, using upstream one (used on Debian
package).