player: some improvements about the player

[cosven]

1. stop the player immediately when user plays a song, so that user knows the action is handling
2. remove some misleading logs from the playlist module
3. fill the cover_label with random color when cover is not available

[coveralls]

Pull Request Test Coverage Report for Build 7158504609

• 45 of 79 (56.96%) changed or added relevant lines in 7 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.06%) to 57.191%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
feeluown/library/models.py	1	2	50.0%
feeluown/gui/components/line_song.py	5	8	62.5%
feeluown/gui/uimain/player_bar.py	3	6	50.0%
feeluown/player/playlist.py	19	22	86.36%
feeluown/gui/widgets/cover_label.py	4	9	44.44%
feeluown/gui/drawers.py	11	30	36.67%

Files with Coverage Reduction	New Missed Lines	%
feeluown/gui/drawers.py	1	50.63%

Totals
Change from base Build 6946375179:	0.06%
Covered Lines:	9031
Relevant Lines:	15791

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💛 - Coveralls