Fix install icons and desktop-file for freedesktop standarts

We will install GameConqueror.png w/ different sizes to hicolor
theme and use Icon=GameConqueror in desktop-file.

Freedesktop specifications:
http://standards.freedesktop.org/icon-theme-spec/latest/ar01s07.html
http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#recognized-keys

Reference BUG: http://code.google.com/p/scanmem/issues/detail?id=68

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>