Support 12bit IVs on encrypted httpupload files

Small bugfix for windows users and fixed some demandimport errors. Added correct license informations to the source files, too.

Added ability to mark text including image and use copy'n paste to paste this text including the original url. Also removed the encryption settings from filename in save as dialog.

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