fix #232931: allow breaks to be added by shortcut on any selection

[MarcSabatella]

Came up in the forum and seemed like a good suggestion, easy to implement. So now you cna hit enter while still in note input mode and enter your line breaks as you go. You can also add a line break by keyboard with a measure selected, not just a barline. You can also multiple line breaks at once by list selecting multiple barlines.

Only question was how best to handle a range selection of multiple measures. In this PR I went with the same approach we use for clefs, key, and time signatures: insert before and after the selection. The effect is, you can select a range, it Enter, and it will all be fit on a single line if possible.

[IsaacWeiss]

Does this also change the behavior of double-click, which currently applies it to each selected measure?

[MarcSabatella]

No it does not. Double click does not use the "command" assigned to this but instead works like drag & drop. It would in principle be possible to special case it in the double click code as well. Eventually I'd love there to not be such different code. We might move closer to that as a result of the accessibility work going on for GSoC.

[IsaacWeiss]

Yeah, not a fan of inconsistencies... everything else I can think of that can be found in the palettes and also has a shortcut (hairpins, slurs, grace notes, certain articulations) works exactly the same way when applied to a range selection by either method.

[lasconic]

Let's try it in master. It has conflicts in 2.2, so let's try it first in master and then make PR for 2.2 if we think it works ok.

[MarcSabatella]

Thanks. FWIW, the code should be close to identical for 2.2, it just needs to be moved to the main cmd() function which is where this gets handled - there is no separate cmdToggleLayoutBreak() function.