Add the new Keywords entry to the desktop file.

It was added recently to the xDG specs
http://standards.freedesktop.org/desktop-entry-spec/latest/ar01s05.html
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It should improve the discoverability of pcsx2 for gnome shell users in any
distro and Ubuntu users at the very least. It's unrelated to the menu
hierarchy so it does not need a special hack for OpenSuse. Also it should
not break anything.
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It's an optional entry but the "gnome guys" really want software to use it
more.
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It should be concise and not the name of the software so I just did
game;emulator; since for example gnome-chess uses game;strategy;.

linux_various/pcsx2.desktop.in

@@ -7,4 +7,5 @@ GenericName=Playstation 2 Emulator
 Comment=Sony Playstation 2 emulator
 Exec=pcsx2
 Icon=pcsx2
+Keywords=game;emulator;
 Categories=@PCSX2_MENU_CATEGORIES@