Added right click menu to preview plugin and reorganized code. Also fixed regression regarding thumbnail size and added gtk pixbuf thumbnail generation if pillow isn't available.

warning: this can take very long (about 1 minute for 1 MB)

New version using GTK eventbox instead of xhtml (looks cleaner and also provides the url in copy/paste operations)

 * Content-type and content-length are checked for every URL in MUCs and regular chats
 * Plugin config now have a setting for max image download size (which is checked to content-length)
 * If content-type match and content-length is smaller then setting the image will be downloaded and displayed
 * If image can be loaded, the URL is no longer displayed (except as a tooltip on the image)
 * Image is clickable (opens browser with URL)
 * Images are displayed using XHTML
 * URL is displayed until image is displayed
Fixes #102, #105