segmented_button: support variable-width buttons when horizontal width is Shrink

[mmstick]

Once this is merged, I will make PRs for cosmic-edit and cosmic-term to utilize the new features.

When the width of a horizontal segmented button is set to Shrink:

• Enable variable width button widths
• Supports tab pagination when there is too little space

Also makes the following improvements:

• Drawing should now require less CPU
• Fix for drawing in vertical layout

Remaining issues to resolve

• Integrate better with a tab add button
• Fix overlapping layout when Fill width when pagination is enabled