Fix issue #23985.
Furthermore implements deleting all the measures covered by the multi-
fix warnings due to -Wunused-parameter
handle style change for Marker
fix warnings due to -Wunused-variable
fix warnings due to -Wswitch
fix warnings due to -Wparentheses
fix warnings due to -Wunused-function
fix warning reg. "name xxx already in use"

using the names they defaults to
fix error introduced when fixing warnings due to -Wswitch
fix error introduced when fixing warning -Wunused-function
fix cmake warning with cmake >= 2.8.11
fix warnings due to -Wunused-but-set-variable
getting rid of some useless window titles

these are all identical (and as such not helpful to tell them appart),
don't seem to be shown anywhere but are all marked for translation
Fix for issue #24339
Slurs between voices will now save and restore properly.
The MuseScore debugger has not been fixed to display the cross-voice
property of such slurs.
fix 24359: changing "directory" to "folder"
fix #24350: markings lost after timesig change

Annotations were completely lost, spanners placed incorrectly.  Also,
repeat measure symbol caused hang.  These are fixed.  Key signatures
still can get messed up, at least until reload.  And barline styles are
lost even when they coincide.
fix #24359: changing plural to singular
fix 24359: it wasn't just the UI file affected...
fix 23366: more capitalization
add test