Fix ScrollBar mouse_event handling with ListBox

[skimmmer]

Propagate handled from ScrollBar if original widget (ListBox) handled the event.

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment

Description:

If original widget (i.e. ListBox) handles the event, return True indicating that the event was handled.

Fixes #904

[coveralls]

Pull Request Test Coverage Report for Build 9477495461

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.741%

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Totals
Change from base Build 9463006499:	0.0%
Covered Lines:	9412
Relevant Lines:	12740

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💛 - Coveralls