Fix missing semi-colon in desktop file

When packaging the latest version of this project for Fedora, I get the following errors 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
    /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 key "Categories" in group "Desktop Entry" contains an unregistered value "Accessories"; values extending the format should start with "X-"

This pull request simply adds in the missing semi-colon and since "Accessories" is not a registered main category, I have changed it to "Utility" which is synonymous with "Accessories." Please see the desktop entry specification for information about the registered main categories:

http://standards.freedesktop.org/menu-spec/latest/apa.html#main-category-registry