Fix #10744 (crashes with CSV files with parallel rendering)

I can't replicate the crash, but the static QRegExp instances will not work
with multiple threads.
Fix #10828 (slow project loading)
When grouping layer tree nodes, insert new group at the same position
Fix bundle-relative linking for libqscintilla2.dylib (now required lib) on Mac
[FEATURE][composer] Allow setting attribute table header and content font colors. Fix saving alpha channel for grid colors.
avoid initially loading value map previews from the first layer (might be expensive)
add msvc notes
[TRANSUP] string update by request

Signed-off-by: Werner Macho <werner.macho@gmail.com>
Legend refactoring: first steps towards legend customization

- map layers have associated QgsMapLayerLegend instance
- QgsMapLayerLegend is interface for generation of legend with some default implementations
- QgsLayerTreeModelLegendNode is interface for representation of legend item in layer tree model with some default implementations
[composer] More cleanups - move font utils from QgsComposerItem to QgsComposerUtils, add tests
[composer] documentation improvements