Staff type editor: Fix disambiguation of parameters common to several groups.

- GenKeySig and ShowLedgerLines are shared by standard and percussion group
- SlashStyle is shared by all three groups

When saving the style, the current implementation overwrites the shared parameters with percussion params.
partial fix for #22996
Merge pull request #906 from mgavioli/Fix_stafftype_common_parameters

Staff type editor: Fix parameters common to several groups
fix #12079
fix 'German accent'
add comment
make it C++ and don't translate

translating doesn't work (not this way at least) and won't happen anyway
as the translation mechanism is set up only later in the code, after the
use of the usage() function.
fix formatting
small optimization: call spatium() only once
ignore some backup- and temp. files

for vim and QtCreator
plugin documentation, consistency
fix plugin documentation

attempt(!) to prevent FileIO to show up twice in the help for plugins,
but this is the RightThing anyway, no other such marking in any other
.cpp files, only in .h
improve mtest

unified diffs are much easier to read
plugin documentation

taking stuff out off the plugin doumentatation that is irrelevnat there,
but leaving it in a way DoxyGen can still get it.
some more improvements to plugin documentation
fix stray rest
update file format history

as per http://musescore.org/en/developers-handbook/version-information
again plugin documentation cleanup
remove a couple unneeded Q_ENUMS
expose GlissandoType