Add proper error handling when uploading a file
Simplify logic in UploadProgressFunctor
Fix a memory leak.

We need to collect connections objects from time to time if not memory
usage will grow infinitely.
Use a macro list to reduce code duplication in EnginioString class.
Make ImageModel not editable.

By default the view is showing a line edit, which looks strange and
images are not editable anyway.
Merge pull request #169 from nierob/smallImprovements

Add proper error handling when uploading a file