[5.1] Allow custom fields form be manipulates like category form

[bembelimen]

Summary of Changes

Allow a component to manipulate the custom field form like you can do with category view

Testing Instructions

• Apply patch
• Open a field form from com_content in the backend
• Check that you get the form here and can manipulate it.

$form->getName() should give com_fields.field.com_content.article

$form->removeField('display', 'params'); removes the "Automatic Display" field

Actual result BEFORE applying this Pull Request

No way to manipulate the form via PHP directly

Expected result AFTER applying this Pull Request

You can manipulate the form

[brianteeman]

would love to test this if you could provide some realistic test instructions

[bembelimen]

would love to test this if you could provide some realistic test instructions

Done

[brianteeman]

would love to test this if you could provide some realistic test instructions

Done

I will have to pass on testing this then

[chmst]

I have tested this item ✅ successfully on 343863b

Tested following the instruction and was able to manipulate coustoms fields.

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[HLeithner]

I have tested this item ✅ successfully on a86650f

Tested with com_contact, call works site didn't break

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[richard67]

I've restored @chmst 's test result in the issue tracker since the commit which has invalidated the test counter was just a clean branch update.

[richard67]

RTC

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[LadySolveig]

Thank you! And also for support and testing @chmst @HLeithner @richard67