Support & demo interrupt handlers for buttons

[Gadgetoid]

This demo - inspired by #5 - attaches an interrupt handler to the Display HAT Mini buttons, and then feeds them into the PyGame event queue so they can be read as ordinary keys.

Buttons could generate pretty much any event, but I picked the a/b/x/y keycodes since these match the actual buttons.

[coveralls]

Pull Request Test Coverage Report for Build 1790108513

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.6%) to 49.398%

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Files with Coverage Reduction	New Missed Lines	%
displayhatmini/init.py	3	75.93%

Totals
Change from base Build 1789624486:	-0.6%
Covered Lines:	41
Relevant Lines:	83

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💛 - Coveralls