Style Score/Part tabs like Panel tabs

[eakwarren]

Resolves: #23377
Style Score/Part tabs like Panel tabs. Specifically 3 changes:

1. heights are now consistent
2. background color states are now consistent
3. active score/part tab text is now bold, matching panel active state.

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[cbjeukendrup]

We've agreed internally that we don't want to make the height change; the height is intentionally not the same as that of the dock panels, to visually distinguish both tab bars (and, as I would personally add to that, vertical space in the score view is precious). So, let've move on with #30794, for which I've asked final approval from the design team.