Formfield layouts

[phproberto]

Summary of Changes

This is a resubmission of #13018 because I totally abandoned @Spudley which is the worst I could do with a new contributor :( I'm really sorry mate and I hope you get your contribution merged at least 2 years later. I created a custom email rule for the repo notifications and everything is messed there.

This pull request adds the default layout paths to FormField::getLayoutPaths() so developers can inherit them and add any custom layout path required by their extensions.

Before this patch a developer wanting to set custom layout paths had to do something like:

	protected function getLayoutPaths()
	{
		$renderer = new FileLayout('default');

		return array_merge(
		    array(__DIR__ . '/custom-dir'), 
		    $renderer->getDefaultIncludePaths())
	}

After this patch it should be easier like:

	protected function getLayoutPaths()
	{
		return array_merge(
			[__DIR__ . '/custom-dir'],
			parent::getLayoutPaths()
		);
	}

which should be better because:

• Parent cares about default include paths.
• Child classes do not have to explicitly create an instance of FileLayout

This also adds unit tests for FormField::getLayoutPaths().

Testing Instructions

Edit any form containing fields (for instance com_content article) and ensure that form fields are shown correctly.

If you want ot test layout paths customisation you can create a field like it's done in the tests:

https://github.com/phproberto/joomla-cms/blob/formfield-layouts/tests/unit/suites/libraries/joomla/form/_testfields/customlayouts.php

[chmst]

I have tested this item ✅ successfully on 683cd61

Tested following the testing instructions, on a localhost, xampp, win10, and the testfield.

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[zero-24]

I have tested this item ✅ successfully on 4b6c9fc

Working as intended leaving this to @HLeithner for an merge.

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[Quy]

RTC

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[HLeithner]

thx

[Spudley]

thank you 😄