Thumbnails, notifications and icons!

Yay! Many changes have been made:
- thumbnails are now generated (not yet displayed)
- reworked the contentmanager to do IO operations in separate threads
- contentmanager now also nicely manages the thumbnails (TODO: on re-encryption, recreate thumb)
- single and multi file selections modes are now much more user friendly and act according to ones expectations
- added toast feedback to notify the user of the successes/failure of operations in the app
- original files get removed, but if not possible to delete, user will get toast that says to remove the file manually TODO: can we somehow work around this deletion problem? cant we throw a delete intent to the system or whatever? having trouble with the images in the camera folder.

My excuses for doing all this in 1 branch, but I just couldn't wait for a pull request to be accepted merged to the master and I wanted to work with the stuff from this branch. I'm new to git, so please tell me how I should have dealt with this problem?