Fix #15761: Change Behaviour when clicking "Apply to style" property with different selected elements

[pacebes]

Resolves: #15761

Change behaviour when applying to style. "Save as default style for this score" is only active when selecting several elements if:

• Property style is the same
• Property value is the same

• I signed the CLA
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• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
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• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)