Update ProcessTemplate so that it automatically saves custom properties in the way ProcessField does

[Toutouwai]

If you hook into ProcessField and add a custom inputfield to the edit form, the value of that custom inputfield is automatically saved to a custom property of the Field. That is very handy and is thanks to ProcessField::saveInputfields($wrapper), which goes through all the inputfields in the wrapper and sets the values to the Field.

But ProcessTemplate doesn't follow the same approach in executeSave() and instead will only save properties with names that are predefined in the method, for example: $properties

This makes adding custom properties more difficult as you have to hook into the executeSave() method as well as the buildEditForm() method. Perhaps ProcessTemplate could be updated so that it follows the same approach as ProcessField?