skins: add Slider bars

[ronso0]

...so that the knob arcs #2686 don't feel so posh :)

Bars can be used alone or in conjunction with slider graphics.
When used alone, experimtent with <Size>, <BarMargins> to yield a proper (read: tolerant) touch/click-responsive surface.

Note: Margins are absolute values, so if you want to use graphics AND dynamic sizes (stretched graphics) it won't work well as of now.

<SliderComposed>
  <TooltipId>channel_volume</TooltipId>
  <Size>46f,107f</Size>
  <Horizontal>false</Horizontal>

  <BarColor>#ff8800</BarColor> any valid hex or rgba color
  <BarWidth>4.0</BarWidth> width in pixels
  <BarMargins>9,9</BarMargins> bar start,end. left,right / top,bottom respectively
  <BarAxisPos>23.0</BarAxisPos> shifts bar from left or top
  <BarBgColor>#050505</BarBgColor> enables background bar
  <BarBgWidth>8.0</BarBgWidth> optional, adopt <BarWidth> if empty
  <BarBgMargins>7,7</BarBgMargins> optional, adopt <BarMargins> if empty

  <Connection>
    <ConfigKey><Variable name="group"/>,volume</ConfigKey>
  </Connection>
</SliderComposed>

ToDo

• remove demo styles

[ronso0]

Let me know when you consider this ready for merge. I'll remove the demo styles then.

[Holzhaus]

Code looks good so far, I'll test this tomorrow.

[Holzhaus]

Thank you! Looks great. LGTM (except for the small comments I posted above).

[ronso0]

Okay, cool.
I'll force-push this without demo styles and with some simplifications, for example just adopt the the main bar's width/margin if a valid color is set for the background.

[Holzhaus]

@ronso0 Ready?

[ronso0]

@ronso0 Ready?

Yup!