Use QgsRasterTransparencyWidget in raster properties dlg

[elpaso]

Remove a bunch of duplicated code and fix #44949

[elpaso]

CC @DelazJ , I'd appreciate if you could test this one.

[DelazJ]

@elpaso I downloaded the Windows artifact but my Windows is somehow blocking me from running it 😠 (security blah blah) and when deactivated I can't pass the "Setting up the GUI..." step.
Will test in the next build

[DelazJ]

@elpaso tested and I think I found an issue: the layer styling panel is not automatically updated if you do the change in layer properties dialog

[elpaso]

@DelazJ if this issue was already present before my patch can you please file a separate issue?

[DelazJ]

No chance it could have popped up given that the "display nodata as" color widget was absent in layer properties before your patch (this was the reported issue, besides the code duplicate Nyall noticed). But I opened #45132 for reference.

[elpaso]

Yeah, of course. I should have read your comment more carefully 🙃.

It is probably unrelated to my patch though.