Tidying up colors and prettify frames

[mike-spa]

Resolves: #26171

At the moment we have the following color definitions:

• Formatting color (defaults to bright pink)
• Frame color (defaults to grey)
• Invisible color (defaults to another grey)

Some items need a grey color, but that color is not conceptually related to either invisible elements nor frames, and it shouldn't change if users for some reason decide to change frame color or invisible color from the preferences. So I've introduced a further color definition, called

• Score grey color

which defaults to the same grey as the frame color but is not editable (in other words, it's hardcoded).

Here's a summary of the whole thing.

• Selected grip handle: formatting color ❌ ➡️ Changed to score grey
• Slur/tie editing lines: formatting color ❌ ➡️ Changed to score grey
• Irregular measures: formatting color ✅
• Layout breaks / System locks / Spacers: formatting color ✅
• Staff type change: formatting color ✅
• Atonal key signature in palette: formatting color ❌ ➡️ Changed to score grey
• Page margins: formatting color ❌ ➡️ Changed to score grey
• Page number in navigator: formatting color ❌ ➡️ Changed to score grey
• Text editing frame: frame color ✅
• Frames: frame color ✅
• Figured bass extensione lines: frame color ❌ ➡️ Changed to invisible color

While I was at it, I took the opportunity to try to prettify frames by

• Slightly decreasing the line width (now at 0.1 staff spaces)
• Increasing the dash size
• Using round caps and round joins

[jessjwilliamson]

Awesome!