Separate use cases and capabilities

Reorganize the document structure so that
capabilities aren't grouped under use cases.
Instead each use case has a list of required capabilities
(which will eventually be used to auto-generate
a list of supported use cases for each capability).
For widgets, use cases are further divided
by whether they primarily support the author
or the end-user.

As a consequence, the "abstract use case"
of a generic map viewer widget has been removed.
Other sections have been slightly modified and moved around.

Closes #10

Author: AmeliaBR