instruments.xml update

Electronic instruments is now complete, the marching percussion group
has been added, some anomalies caused by use an <init> tag which is not
part of the DTD addressed. Checked for XML correctness and all appears
to be well.
fix potential crash when reading a linked staff
regressions: fix slur playback
move slur properties to inspector
partially fix regression tests
add gonville font in TTF format
replace OTF font by TTF font, fix PDF size issue #21567
fix tests
Remove a left-over from removing guitar string data as default for each instrument.
fix #21730 + add compat test for accidentals
fix grace note layout in beam context
Merge pull request #396 from mgavioli/remove_default_guitar_stringdata

Remove a left-over from removing guitar string data as default for each instrument
fix cursor position in note entry mode
fix: when switching tabs, score position moved
tuning voice buttons
fix drag&drop of images
fix #19427, fix image drop to note
regression: fix note line
update light theme, default color for voices, layoutbreak, background and foreground
cosmetic changes, color for frames,...