* Files outside the project directory now show up with a much more logical
name in the IDE. External files have a link target which is relative to the
root xamarin-macios directory, and files included from the mono archive show
up as such as well.
* Rename a few mtouch/mmp-specific files to contain 'mtouch' or 'mmp' in the
filename, to avoid having multiple files in the projects with the same name
(it's confusing every time you search for a filename in the IDE and get
multiple filenames where only the directory is different).
* Add a tools.sln that contains only the mtouch and mmp projects. This makes
it easier to work with both mtouch and mmp and the same time, while not
making VSfM unbearably slow by loading many projects in the same solution.