Gui Toolkits

Hi.

While I was navigating in the website, I noted the breadcrumb nav for the GUI lead to [this page](http://www.mono-project.com/docs/gui/) with a TODO for the Toolkit list.

But there was already a very complete list at the [Choose a GUI Toolkit](http://www.mono-project.com/docs/gui/gui-toolkits/)* page. To avoid duplicate content, I splitted the Choose a GUI page in two:
- Moved the list to the index for the GUIs.
- Kept the guide in the "Choose a GUI"

There are cross references between the pages too, so a reader can still read both.

\* The link to this page at the Overview says "Choose a GUI Toolkit" but the title of the page is "GUI Toolkits". I fixed that too.
Underlining with `---` has the same effect as prefixing the title with `## `. I personally prefer the underline as it provides more visual separation.