[Avatar] Make it based on RoundButton and change the visuals

Change properties to use shorter names that don't all start with the same character.

Set default border width to 0

Set default border color to match Separator's color

Make initials always uppercase. This makes it much easier to center the text.

Make the implicit size, padding and inset work like it normally does in QQC2 components.

Change the way text is sized to be based on availableHeight, but without using Text.Fit.

Re-enable text at small sizes. It should look OK now.

Use QtRendering so that avatars can be animated without distorted text.

Set the font weight to Medium. This looks nice and also prevents QtRendering from looking significantly different from NativeRendering.

Add behavior for when root.flat is true. flat changes the shadow size and whether or not there is a gradient.

Disable mipmapping. It made images blurry.

Enable asynchonous image loading to improve load times.

Use OpacityMask effect instead of ShadowedTexture. The latter would need access to currently private Qt APIs in order to not distort images.

Rearrange the contentItem and background properties to be like they commonly are in QQC2 styles.