Fix missing semi-colon in desktop file

When packaging the latest version of this project for Fedora, I get the following error when validating the desktop file:

    + desktop-file-validate /home/mbooth/rpmbuild/BUILDROOT/gnome-shell-extension-pomodoro-0.10.0-1.fc20.x86_64//usr/share/applications/gnome-pomodoro.desktop
    /home/mbooth/rpmbuild/BUILDROOT/gnome-shell-extension-pomodoro-0.10.0-1.fc20.x86_64//usr/share/applications/gnome-pomodoro.desktop: error: value "GNOME;GTK;Accessories" for string list key "Categories" in group "Desktop Entry" does not have a semicolon (';') as trailing character