ProjectManager: active task is not saved between sessions

[diegoiast]

When you restart qtedit4, the last used task is forgotten, instead the first task is active.

[diegoiast]

Active task should be a project state. It does not belong on the json file - as this defines a project definition. The reason for this - is that we cannot commit such change to the git repo, otherwise - developers will not be able to save personal preferences.