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.
fix #24305
fix #24468
fix #20082
fix chordline regression
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
Score file reading: fix bar line span for non-5-line staves.

While reading a score, if a staff is found with number of lines != 5 and no explicit bar line span set (i.e. default bar line span), the bar line span remains set as for a 5-line staff.
fix #21168
Fix StaffType::operator==

For TAB_STAFF_GROUP, the equality operator does not check all the member variables (does not call isSameStructure())

Also, simplified some duplicate tests.
fix #12112
fix 'German accent'
ignore some backup- and temp. files

created by vim and QtCreator
fix stray rest
add a comment

same as the one in the correspoding .h file
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.
Remove the mention of an option that (no longer?) exists
fix #25755: avoid duplicates in plugin help

and add two missing ones. Also remove a 'plugin comment' from a .cpp
file, where it doesn't serve any purpose anyway (but at first tricked me
into believing this were the the culprit for the duplicate)
improve mtest

unified diffs are much easier to read
update file format history

as per http://musescore.org/en/developers-handbook/version-information