Provide workflow actions to hold transitions based on form status.

[rharkins]

Add ability to get at information from the forms and hold up progress if certain fields do not contain appropriate values.

[rharkins]

This should help do some of what is described in #3260.

[hasienda]

(In [11283]) TracFormsPlugin: Add optional write protection to input fields, refs #3264, #5353, #8715 and #9640.

A new keyword argument '-mode', for now exclusive to tf.input and tf.textarea,
with value to choose from the following list:

• ro - input blocked, like static text, but in non-editable form field
• rd - same as 'ro', but don't display stored value but default instead
• rw - regular input behavior at all times (default)
• once - initially 'rw', but blocked ('ro') after first form submission

It's an optional argument, retaining full backwards-compatibility
with existing field definitions by choosing -mode=rw as implicit default.