Indent wrapped lines

Indenting of wrapped lines can now be turned on in the Preferences
window. So far, the indentation does not adapt to the number of tabs at
the beginning of the line.
Put hanging indent on tab-indented lines as well

The hanging indent needs to adapt to the number of tab characters at the
beginning of each line. This is done using GtkTextTags named indent-1,
indent-2, etc. that override the global hanging indent settings of the
text buffer. When inserting or deleting text, the relevant portion of
the text is re-scanned and these tags are re-applied.
Merge branch 'indent-wrapped-lines'