Fix #25165 - crash when changing rest measure duration to or from irregular time.

When the actual duration of a measure is changed and either the starting or ending duration cannot be represented with a simple value (for instance 5/4) and the measure contains a single whole measure rest, a division by 0 results. See http://musescore.org/en/node/25165 for examples, discussion and analysis.

Fixed.

If the new measure value is different from the current time sig, rest(s) to represent the actual measure values are used instead of whole measure rests.
Tick where to place the newly added rests was wrong.
fix build of plugins manual
Added support for voltas in Guitar Pro 4 files
Support for voltas in Gutiar Pro 3 files
fix #22898 - tests to verify voltas are being imported correctly in Guitar Pro files
test to fix #25559: show splash screen on linux
fix #25553
Merge pull request #809 from mgavioli/fix_25165_pickup_meas_with_irreg_metre_measure_rest_crashes

Fix #25165 - crash when changing measure with whole measure rest
Merge pull request #867 from Jojo-Schmitz/plugins-manual

fix build of plugins manual
Merge pull request #869 from triolenzunge/25559-showsplash

test to fix #25559: show splash screen on linux
Merge pull request #868 from jpirie/22898-guitar-pro-volta

Volta support in GP3 and GP4 files
moved fontconfig in mtest build
Merge pull request #871 from BartlomiejLewandowski/fontconfig

moved fontconfig in mtest build
fix #25278: chords won't overlap barline
correct measure width calculation
add style parameter to control space to barline
fix default & add test
rebase, minor code cleanup