First good usable set of actions

This is the first release I would consider being stable and feature-rich enough to use.

There are currently actions for:

• rotating images
• shrinking images
• grayscaling images
• removing alpha channel from images
• converting images to an A4 multi-page PDF document
• shrinking PDF file sizes
• de/encrypting arbitrary files with GPG

Everything with a beautiful progress bar realized with zenity.