Remove isValidValue in favor of reusing validateFilter

The isValidValue method was too general and did not handle branching
filters.

The validateFilter method has been extended to support validating without
displaying the error message by passing the silent: true option. Assuming
each control implements a validate method, this will work as expected.

Fix #546

Signed-off-by: Byron Ruth <b@devel.io>
Merge pull request #550 from cbmi/issue-546

Remove isValidValue in favor of reusing validateFilter