All widgets copy&pasted the same description, including an invalid reference
to "Layout", which should be "Elm_Layout".
SOME of them had been fixed over the years but this commit fixes all of them.
This significantly reduces the number of Doxygen warnings and adds meaningful
links to the docs.
This fixes compilation for terminology for example.
Thanks to @Etrunko for the report.
Note: My builds originally worked fine because my installed
include folder contained the old files :(