The lexer needs its own lexerState, before it was using
the syntax highlighter user state. This breaks the brace
depth calculation of the highlighter.

Reviewed-by: Roberto Raggi

Done-with: Thomas Hartmann

Done-with: Erik Verbruggen

See BaseTextEditor::setRefactorMarkers(...). The markers support
a clicked signal via BaseTextEditor and tooltips. They feature
a cursor, but are always positioned at the end of the line (not block!).

As special gimmick they do extend the document width when they
are positioned outside the document area.

This is a "back-port" for 57f2b3e4 from master
into 2.0.

Done-with: ckamm

The new and cleaner foldingIndent in the block user data will
make it easier to support other kinds of indentation for various
other programming languages (like Python).

The Parentheses, TextBlockUserData and TextEditDocumentLayout classes
and their member function implementations were spread around the
BaseTextEditor class. Moving them to their own file to make the code a
bit better organized.

Reviewed-by: mae