Adjust horizontal centering spacing for rest

[sbonaime]

Description of Change(s)

Adjust horizontal centering spacing for rest

Fixes Issue(s)

Fix #540

[cameronwhite]

Thanks for looking at this!

While doing some testing I noticed that the new spacing didn't look as good for some durations like half rests, so perhaps the alignment needs some different settings per type

[sbonaime]

Maybe we should takie into account the width and height of each symbol before placing them. Or maybe at the drawing stage ?
Few examples were symbol width is important :

Tremolo picking

Vibrato

Large Vibrato and vibrato

[cameronwhite]

Yeah, there should ideally be some adjustments depending on the symbol

I recall it being hard to get accurate information about the width of each symbol from the Qt font info classes, which is why there are a lot of hardcoded scale factors in the layout code.