[MU3] Style Dialog improvements

[cbjeukendrup]

Resolves: https://musescore.org/en/node/314646 = https://trello.com/c/TyNjUPAH/79-format-style-page-and-format-style-score-dialogs-broken

• Fix #314646: Fix sizing of some pages in the Edit Style Dialog
  And make sure it will also work on smaller screens by using a Scroll Area for the most critical pages.

• Style dialog: reduce width of some pages and add some more scroll areas
  To make sure it also looks and works nice on smaller screens and to prevent the side bar from becoming wider and smaller for each page.
  And the Tab order is cleaner than ever before!

• Show Style Dialog always non-modally and bring it to front if already open (Follow up for PR Fix #302990: Set Style Edit dialog to be non-modal to allow score traversal when s… #6845)
  In Format > Style…, the dialog was shown non-modally, but when you opened it from a context menu or by double-clicking the header or footer, it was shown modally. Now, it will always be shown non-modally.
  When the dialog was already open but in the background, and you did something to open it again, nothing would happen. Now, the dialog will be brought to front in that situation.

• Fix warning about icon file not being found
  At some places in the Style dialog, the name for the reset icon was specified without the :/ prefix, like data/icons/edit-reset.svg instead of :/data/icons/edit-reset.svg. That produced a runtime warning (but remarkably, the icon did show up correctly).

Here is a GIF showing all the pages of the style dialog:

And here is a capture of the dialog on my 21,5" screen (4096 × 2304), to get a better idea of the size:

If necessary, the user can resize the dialog even further down.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• I made sure the commit message title starts with "fix #424242:" if there is a related issue
• I created the test (mtest, vtest, script test) to verify the changes I made

[Jojo-Schmitz]

I hope you left the tab order intact (or fixed it again)?
Edit: I see you mention it in the initial PR message.

[cbjeukendrup]

I hope you left the tab order intact (or fixed it again)?
Edit: I see you mention it in the initial PR message.

Yep, don't worry!