app icons: name the app icons with the application id

Fixes #77

data/org.gnome.gThumb.Import.desktop.in.in

@@ -6,7 +6,7 @@ Comment=Import the photos on your camera card
 Categories=GNOME;GTK;Graphics;Viewer;RasterGraphics;2DGraphics;Photography;
 Exec=gthumb --import-photos %U
 # Translators: Do NOT translate or transliterate this text (this is an icon file name)!
-Icon=gthumb
+Icon=org.gnome.gThumb
 MimeType=x-content/image-dcf;x-content/image-picturecd;
 NoDisplay=true
 StartupNotify=true

data/org.gnome.gThumb.desktop.in.in

@@ -8,7 +8,7 @@ Keywords=Image;Viewer;
 Categories=GNOME;GTK;Graphics;Viewer;RasterGraphics;2DGraphics;Photography;
 Exec=gthumb %U
 # Translators: Do NOT translate or transliterate this text (this is an icon file name)!
-Icon=gthumb
+Icon=org.gnome.gThumb
 MimeType=image/bmp;image/jpeg;image/gif;image/png;image/tiff;image/x-bmp;image/x-ico;image/x-png;image/x-pcx;image/x-tga;image/xpm;image/svg+xml;image/webp;
 StartupNotify=true
 Terminal=false

gthumb/gth-browser-actions-callbacks.c

@@ -144,9 +144,9 @@ gth_application_activate_about (GSimpleAction *action,
 				    NULL);
 
 	logo = gtk_icon_theme_load_icon (gtk_icon_theme_get_for_screen (gtk_widget_get_screen (GTK_WIDGET (window))),
-					 "gthumb",
+					 "org.gnome.gThumb",
 					 128,
-					 GTK_ICON_LOOKUP_NO_SVG,
+					 0,
 					 NULL);
 
 	gtk_show_about_dialog (GTK_WINDOW (window),