[5.3] [Events] Use event classes for custom fields plugins

[Fedik]

Summary of Changes

Update fields plugins to use SubscriberInterface and event classes.

Testing Instructions

Create/edit custom fields, all should work as before.

Actual result BEFORE applying this Pull Request

Works

Expected result AFTER applying this Pull Request

Works

Link to documentations

Please select:

• Documentation link for docs.joomla.org:
• No documentation changes for docs.joomla.org needed
• Pull Request link for manual.joomla.org: New Event classes Manual#177
• No documentation changes for manual.joomla.org needed

[exlemor]

I have tested this item ✅ successfully on 6295367

I created a Textarea field, a radio button field, Media (image) and it worked perfectly.

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[Frostholm7]

I have tested this item ✅ successfully on 6295367

with new installation with 5.1 dev and 5.2 dev
Have I tested
New Group
New Subfild
New calendar
New Articles
Filled in the new fields
Saved
Checked logs
everything ok

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[Quy]

RTC

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[Hackwar]

I'm not convinced that the subform field works and since none of the testers claimed to have tested that, I'd like to wait with merging.

[exlemor]

@Hackwar - I just retested it with the subform field... Here is a screenshot:

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[exlemor]

Here it is (had to use GitHub, Joomla Issue Tracker wouldn't let me add the image :/

The subform field appears at the top and the regular custom field on the bottom.

[HLeithner]

This pull request has been automatically rebased to 5.3-dev.