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 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,...
dotted line for frame
Merge pull request #395 from ChurchOrganist/Instruments.xml-update

instruments.xml update
Fix for Issue #21739

Tablature information has been added to the Electric Guitar definition
which for some reason had none. MusicXML id information has also been
added which was also missing.
add repeat test for #15502
Merge pull request #401 from ChurchOrganist/Instruments.xml-update

Fix for Issue #21739
use interval tree for list of spanners
fix regression test