Extend the interface of ScriptRepository for removing files

Add two new methods to ScriptRepository to allow removing files locally
and from the central repository.

re #7031
Definition of the unit test for removing methods

Creation of the unit test to ensure the functioning of removing files
locally and remotelly.

re #7031
Implementation of removing files ScriptRepository

Implements the remove and remove_local methods.

re #7031
local empty folder returns status remote_only

a unit test to trigger this effect.

re #7031
Solve the issue of empty folder

It also allows removing local folders recursively

re #7031
Provide the delete functionality to RepoModel

Extend RepoModel to support the delete entry functionality.

re #7031
Add trash icon to Mantid

It will be used for the delete functionality.

re #7031
Delegate to Delete functionality

extends ScriptRepositoryView to provide interface to delete functionality

re #7031
Merge branch 'feature/7031_deleting_scripts' into develop