Minor: Change the launcher button icon from Ubuntu's to Arch's

[xgdgsc]

Just like this:
https://github.com/chenxiaolong/Unity-for-Arch/blob/master/unity/launcher_bfb.png

[City-busz]

I replaced the Ubuntu logo with Unity logo from /usr/share/ccsm/icons/hicolor/64x64/apps/plugin-unityshell.png