Use QBrush instead of QColor on some theme properties

Apparently, we can use QBrush -typed properties in the CSS. This just never occured to me before!
So, this has several benefits. A QColor property only allows a singular RGB value, but a QBrush allows the same plus also qgradients, RGBA-colours and maybe even bitmap patterns. So I'm changing some properties to QBrush, where it makes sense to allow this additional functionality - no need to enable it for simple things like text colours or such.

- Song editor background: instead of the earlier hack with 7 qproperties just to set a limited background gradient, we can use only 2 properties and allow much more flexibility with Qt's own qgradient syntax
- Automation editor: background, graph colour, and the sidebar colour - @musikBear recently complained not seeing the grid through the graph, so transparency can help there, and qlineargradients in the graph can produce very cool visual effects. Grid is pointless to change, it should stay single-colour for now.
- Piano roll: here, I only made the background use QBrush - we don't really have much else here that can utilize QBrush, the notes have their own gradient system... maybe the 2nd colour of the note gradient could be customizable though.

There are probably more places where this change makes sense...