[FEATURE] EXT:form - Allow predefined forms

This patchset enables the integration of predefined forms. An integrator
can define forms - for example within the site package - using
plugin.tx_form.predefinedForms. An editor can add a new mailform
content element to a page and choose a form from the list of predefined
elements.

Example integration:

plugin.tx_form.predefinedForms.someUniqueName = FORM
plugin.tx_form.predefinedForms.someUniqueName {
 ...
}

Resolves: #72309
Releases: master
Change-Id: I79def3cc00812bd5e5006b5226216064cc1b7ac7
Reviewed-on: https://review.typo3.org/45357


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>