[Info View] Allow custom visual controls

Add a 'custom' option to the set of
available visual control descriptors,
to allow components to indicate that
some special component (not in the
given list) will be required for
editing some property.

Supports nasa/mct#192
[Info View] Escape test when visual control is custom

Don't know enough about custom visual
controls yet to test that they work
correctly in the Info View, so for the
time being simply avoid that test case.
[Info View] Add class for custom visual controls

Adds abstract class CustomVisualControl,
which plugins can implement fully if
they wish to expose custom controls in
the Info View. This supports nasa/mct#192
which in turn block nasa/MCT-Plugins#32.

The inclusion of this class in mctcore
is somewhat contradictory to the ongoing
maintenance issue nasa/mct#155, but is
consistent with current packaging. As
future work it may be desirable to move
the full gov.nasa.arc.mct.gui package
out of mctcore and into a Swing-specific
bundle; the addition of this class is
not expected to add any difficult to
that refactoring.
[Info View] Add support for custom controls
[Info View] Refactor visual control method names

Change method names in CustomVisualControl
to be consistent with those used by
PropertyEditor.
[Info View] Test abstract custom control
[Info View] Hookup listeners to custom control
[Info View] Amend test, handle custom control
[Info View] Fix comment (checkstyle)
[Info View] Remove bad cast to String
Merge pull request #209 from VWoeltjen/mct192

[Info View] Support custom visual controls