Tango skin :: hidpi fixes

[ronso0]

I fixed some major bugs appearing when Tango skin is scaled up (see lp:1751290), as well as some minor UX issues

• qss: split 'background' shorthand into 'background-color' and 'image' (otherwise those images stayed at 100%)
• Fx flow graphics (were completely gone, width of 'border-image' was not scaled)
• Effect selector (removed explicit 'size', added background to effect list toggle, fix scaling of WEffectSelector and WBeatSpinBox #1518 also fixes the effects list display)
• Fx parameter row (rebuild, margin is not scaled but was necessary for previous stacked layout)
• beatsize spinboxes, buttons & spinbox in Library
• borders in sampler and preview deck

Screenshot 1920x1080, mixxx & system font at 200%

[ronso0]

Don't merge yet, I need to fix Artist/Time row and Overview.

[ronso0]

Ready to test!

[ronso0]

I just finished this PR.
Besides the scaling fixes, there are a few minor changes:

• FX unit labels and show_parameters toggle have a background now, so they visually relate more to the actual FX unit. Same applies to sampler row expand toggle
• toggle for additional, hidden deck controls (quantize, repeat etc.) now sits at the outside of deck buttons row
• right-hadn decks' Artist/Title are aligned right (fixes A. of lp:1750902)
• I removed Library toggle from the top row. Didn't use it much, Alt+Tab is much quicker to hide tracks and playlists from snoopy fellows

[Be-ing]

LGTM, thanks!

[Be-ing]

This broke the scaling with #1518. I'll revert it in that PR.

[ronso0]

it seems this was merged too early: I just discovered a bug with some button icons after I fixed those to scale well. All pause icons appear blurred, although at least their height matches the buttons' height.

Fixed in #1531