[2.3] - Fix issue introduced in aa0cab2856efbf13b4c0598e97456f076f5e3b6c

In aa0cab2856efbf13b4c0598e97456f076f5e3b6c, i removed the `actions` attribute of some `MODx.FormPanel` derivate, assuming it was used only to process actions, thus being redundant/duplicate with the buttons `process` attribute.

Seems like i was wrong.

This PR intend to fix my error, as well as provide some BC (`actions` are now optional -- & working "nicely" without it -- while still being supported).