Open up some gui elements to theming

[zonkmachine]

Make some elements themeable.

Example: EnvelopeGraph/LfoGraph

/* Envelope graph */
lmms--gui--EnvelopeGraph {
	qproperty-noAmountColor: grey;
	qproperty-fullAmountColor: pink;
	qproperty-markerFillColor: transparent;
	qproperty-markerOutlineColor: black;
}

Example: Step editor, last edited note

Fixes #7306

[Rossmaxx]

@zonkmachine is there anything else left here?

[Rossmaxx]

Haven't tested but LGTM