Add AppMenu

It follows elementaryOS design guidelines: instead of having a
MenuBar, put your settings and stuff like that under an AppMenu
icon on the toolbar.

It has a Settings dialog and a Markdown cheatsheet, both disabled
on the menu as they are not implemented yet (FIXME status). An
About item has already been added. It shows an About dialog when
pressed. Some changes had to be done to some constructors in
order to let the menu access the Application instance.
Add Markdown parsing (emph, strong, H1..H6)

Vala documentation is lame when it comes to understand how
regex and GTK text tagging works. Markdown parsing works by
using some regular expressions (this is the classic way of
handling this), then adding some GTK tags at the begin and
at the end of each match.

I've only added a little subset of Markdown for now, so that
I can work on doing tests and making sure it works. Once I
consider it safe, I'll implement more rules.