Package inport/export functionality corrected/improved. Export was rewriten so that it outputs Categories/widgets/Inputs/... always in the same order as well as notifies the user what it is exporting.

Improved import_package functionality - now import can remove widget's outputs,inputs and options which are not beeing used.

Added import/export functionality for widget definitions (for models: AbstractWidget, AbstractInput, AbstractOutput, AbstractOption, Category).
Three managment functions added:
- export_package
- import_package
- renew_uids
Help included should be enough to understand each functionality.
Everithing is 'quite' thoroughly tested, however, I STRONGLY suggest database BACKUP before doing commands import_package or renew_uids. export_package should be safe.