Add inline code and code block

Inline code is an inline style delimited by backticks (`).
For instance, `this is an example of inline code`. Inline
code is formatted with a different color (because Focus
already uses monospace text).

On the other side, code blocks are preformatted paragraphs.
They appear using the same style than inline code, though.
Codeblocks are composed by lines, each line having a 4 space
or 1 tab indent. For instance:

    This is a codeblock.
    The text inside this paragraph is indented 4 spaces.
    Thus, it's a codeblock.
    Codeblocks may have a tab too instead of 4 spaces.
    I'm not adding a tab, though. I'm alergic to them.
Add blockquotes

Blockquotes are block entities that begin with an > symbol.

> This is an example of blockquote.
> It resembles e-mail blockquotes.
> You can use a blockquote to cite text.
Make window use Gtk.Window.set_application

Didn't know that.