Style schemes: add a scale property

- Add scale / scaleset property to gtksourcestyle objects.
- Pass scale property to underlying gtktextview/textselection with scale property.
- Add code for parsing scale properties from gtksourcestyleschemes.
Add headings styles to def.lang
html: add support for headings styling

- Add support for def:headings1 to 5 for h1 to h5 tags in html
with working attribute highlighting
latex: add support for headings styling

- Map commands part, chaper, section, subsection, subsubsection, paragraph, subparagraph
to heading0 to 6
Doc: scale property for the style schemes