GutterRendererMarks: chain to parent change_view()
GutterRendererLines: recalculate size on view style updated

This fixes a bug with the line numbers, when we change the font size of
the text view (the gutter width was not updated).
Gutter: improve documentation

The FIXMEs will be fixed progressively, as I better understand the code.
GutterRenderer: minor clean-up