Changes include:
* Much better encapsulation of textview/textbuffer, with each isolated
responsibility living independently on its own class/file.
* Less code overall
* Various small fixes around the components involved, such as:
* Indentation of nested lists (fixes#120)
* Unwanted scroll on select all (ctrl+a)
* Removal of unused code around the components involved
* Fixes for scrollbar location, now at the edge of the window
The Theme class encapsulates theme handling, by listing default themes
and providing means to access their gtk/web css.
Besides cleaning things up, it makes it easy to support custom themes
in the future. The user just needs to provide two CSS files, and we'll
be able to instantiate and use a Theme from that.
Gonçalo Silva