applets/kickoff: add option to display only label instead of icon

This commit allows the user of Kickoff on a horizontal panel to
optionally add text to the panel button and/or remove the icon from it.
These options are disabled when Kickoff is located on a vertical panel
because there is not enough room there.

This allows the user to mimic the UI used to open XFCE and Mate's menus,
or GNOME's Activities Overview.