Tools: Be able to change models of tasks and assets widgets

[iLLiCiTiT]

Brief description

Be able to create different source model for assets widget and tasks widget.

Description

In some cases it is better to use different model for assets and tasks widget which is based on different source of data. For example when a tool is querying asset documents for it's purposes then triggers refresh of assets widget. In that case the same data are queried twice which makes loading of tool slower. At the same time a tool may require custom filtering of assets or tasks which is not possible right now. Both use cases would require to create whole new asset or task widget for the tool which is duplication of code.

Changes

• tasks and asset widget have methods to create model and proxy model
  • this way it is easy to inherit from the class and override those methods to define different source of data
• clearing of model of refresh is handled in _refresh_model because if may not be possible for custom model to do so

Note

• This change is needed for 2 or more other features that are long term so it would be hard to maintain those changes

Testing notes:

1. Everything should work as before

[kalisp]

Still working as expected