Rename variables.

* configure.ac
  - Rename from APPLET_GNOME to GNOME_APPLET.
  - Rename from APPLET_GNOME3 to GNOME3_APPLET.
  - Rename from APPLET_KDE to KDE_APPLET.
  - Rename from APPLET_KDE4 to KDE4_APPLET.
* gtk2/toolbar/Makefile.am
  - Rename from APPLET_GNOME to GNOME_APPLET.
* gtk3/toolbar/Makefile.am
  - Rename from APPLET_GNOME3 to GNOME3_APPLET.
* qt3/toolbar/Makefile.am
  - Rename from APPLET_KDE to KDE_APPLET.
* qt4/toolbar/Makefile.am
  - Rename from APPLET_KDE4 to KDE4_APPLET.