Auto-theme improvements

[jansol]

Love the new auto-theme feature, however I normally use dark themes and getting blasted with an almost full screen of pure white upon switching to the next track was not the most fun of experiences so here are some suggestions for improving the feature:

• [ ] Bias color selection towards either "dark" (bright text / dark bg) or "bright" (dark text / bright bg) depending on the user's choice (could be derived from the chosen "main" theme)
• [ ] Bias color selection or do some extra brightening / darkening after choosing colors to ensure the UI text always has sufficient contrast against the current background. Especially against the "current track" highlight the track title can be almost invisible at times.
• [ ] Add an option scale the brightness of auto-theme (background) colors (I like the blurred cover art in the background but it's not visible when the theme color is too bright so I'd prefer backgrounds to only have slight color tint)
• [ ] Fall back to colors from the user-selected theme if track has no cover art (currently it just sticks with the previous auto-theme colors which makes for a somewhat inconsistent UX)
• [ ] Playback control bar and playback time indicator in the bottom right corner should follow auto-theme
• [ ] Menu bar and spectrum analyzer (and presumably VU meter, although I did not test this) should follow auto-theme

[jansol]

Noticed a somewhat amusing "feature" of the auto-theme, namely it seems to swap the colors of artist and track title in the playlist when I move Tauon between monitors.