gui: introduce ClickableMixin

[cosven]

1. click the accordion header can fold/unfold contents
2. click the cover label to show/hide nowplaying overlay (demo)

• add unittest

[coveralls]

Pull Request Test Coverage Report for Build 7687171277

• -31 of 72 (56.94%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.008%) to 55.148%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
feeluown/gui/uimain/player_bar.py	7	9	77.78%
feeluown/gui/uimain/nowplaying_overlay.py	16	22	72.73%
feeluown/gui/widgets/accordion.py	3	9	33.33%
feeluown/gui/helpers.py	12	29	41.38%

Files with Coverage Reduction	New Missed Lines	%
feeluown/gui/widgets/accordion.py	1	29.27%

Totals
Change from base Build 7678294412:	0.008%
Covered Lines:	8635
Relevant Lines:	15658

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💛 - Coveralls