Minor changes to install/uninstall in Makefile.gtk

Changed install to create install directories if they don't exist.
Changed uninstall to remove the icon and .desktop files.
Updated Makefile.qt install and uninstall

Modified Makefile.qt to create install directories if not already
present. Also updated uninstall to remove icons and .desktop files.
Merge pull request #10 from szim90/master

Minor changes to Makefile.gtk and Makefile.qt.