Add peg-markdown-highlight library

Add subproject for peg-markdown-highlight library with qmake project for building it
as static library.
Add link reference to new peg-markdown-highlight library
Fix compile/link for MINGW
First version of highlighting in markdown editor

Add new classes MarkdownHighlighter and HighlightWorkerThread to support
syntax highlighting in the markdown editor.

issue #3
Remove debug statements and nicely end worker thread
Move parsing of syntax highlighting styles

Move parsing of syntax highlighting styles to own new class StyleParser.
This new class also supports retrieving a color palette for the text
editor from the style.

issue #3
Improved theme support

* Organize themes and styles in subfolders
* Automatic change of syntax highlighting theme when
  corresponding css style changes
* replace clearness and clearness-dark with solarized-light
  and solarized-dark

issue #3
Merge branch 'feature-markdown-highlight' into develop

closes issue #3