Allow TextEditor to be used in non-Composer contexts

[clarkwinkelmann]

Changes proposed in this pull request: Modify the drag and drop implementation so that Flarum doesn't crash when trying to use TextEditor outside of the Composer.

Also make the drag and drop target customizable by extensions so that other extensions can make it work in their custom TextEditor locations.

Example implementation that would use the new function https://github.com/clarkwinkelmann/flarum-ext-composer-page/pull/1

Reviewers should focus on: Should be 100% backward-compatible since it only adds new method and none of the code was previously extendable.

Screenshot No visual changes.

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• [ ] Backend changes: tests are green (run composer test).

[GreXXL]

Now that there is a 1.3.0 beta version can this be something more easily merged?

[clarkwinkelmann]

I could add this to another beta release on top of the current beta but I'd rather wait until the beta feature is ready, otherwise it might get perpetually stuck in beta :sweat_smile:

[clarkwinkelmann]

Support now merged. Needs Composer Page v1.1+ and FoF Upload 1.4+ for compatibility