Support for live previews of Pages

[LukeTowers]

This PR adds initial support for a "Preview" tab that presents a live preview of the page currently being edited on the frontend without requiring the page or changes to be saved first.

TODO:

• [x] Dynamically add all fields on the Page form (even those that have been dynamically added by other plugins) to the dependsOn array of the preview field
• [x] Thoroughly test editing all content types
• [x] Test switching layouts
• [x] Ensure that change events are triggered by the richeditor
• [ ] Perhaps make the "Preview" button in the top bar open the preview page in a new tab (will need to make sure that the preview logic does a session::get() instead of session::pull() in that case, could also make the preview button an AJAX request so that the latest form data is forced to be saved to the session before the URL is returned, that's probably the safest bet)
• [x] Improve the styling of the preview tab / controls
• [ ] Improve the Workshop theme's test cases for static pages
• [ ] Adjust the sizing of the iframe per device to be more accurate to the device buttons, perhaps also allow for dynamic zooming of the iframe content window so that it will show exactly how it'll look on the desired device, regardless of the size of the user's screen.

[der-On]

I've created a plugin that allows just that:

• https://gitlab.com/studiobosco/wintercms/pagepreview
• https://packagist.org/packages/studiobosco/wn-pagepreview-plugin

[LukeTowers]

@der-On cool! Do you have a screenshot of what the UX looks like?

[der-On]

@der-On cool! Do you have a screenshot of what the UX looks like?

Sure:

The plugin also allows to add a preview to any model form with a single line of php code in the plugins register method. The model just needs to implement a preview() method.

[jaxwilko]

@LukeTowers, just pushed a commit to prevent the preview being displayed by default so that people have to "opt in" to using it, any thoughts?

Also I fixed some display issues.

Is there anything else that needs doing before this can be merged?

[LukeTowers]

@jaxwilko any reason why it should be opt-in instead of just always enabled?

[jaxwilko]

@LukeTowers not really, just thought it would make sense for people to opt in

[LukeTowers]

Meh, if it's opt in no one's going to see it. If you can think of a reason why someone would want it disabled then we could look at providing that as an option but at the moment I'm fine leaving it always enabled.

[LukeTowers]

@jaxwilko todo before we can merge:

• thorough testing of the editing all content types because of the refactor from controller to objecthelper
• look into the todo (https://github.com/wintercms/wn-pages-plugin/pull/19/files#diff-30c49f6ba6a738fbd3fed3c4e7ddacb319ace9478f52ac56ecc56bec67e32ebfR68)
• remove the config option
• remerge in latest main

[jaxwilko]

@jaxwilko todo before we can merge:

• thorough testing of the editing all content types because of the refactor from controller to objecthelper
• look into the todo (https://github.com/wintercms/wn-pages-plugin/pull/19/files#diff-30c49f6ba6a738fbd3fed3c4e7ddacb319ace9478f52ac56ecc56bec67e32ebfR68)
• remove the config option
• remerge in latest main

@LukeTowers all of there points have now been complete, tested everything and it all looks good, just need to add unit tests

[jaxwilko]

@LukeTowers tests now pass :)