Implement x-posting using x-posting button

[guarani]

Is your feature request related to a problem? Please describe.

Implement UI to add an x-post via the toolbar menu in the Gutenberg mobile editor for sites that have the O2 plugin installed. The UI should be a button that triggers a dropdown allowing selection of a site from the list of available sites.

This is closely related to #2602

Describe the solution you'd like

The design has not been finalized AFAIK, but @iamthomasbishop had mentioned that the x-post button will be part of a popup menu accessed by long-pressing the @-mentions "@" toolbar button.

This will complement the original x-post implementation (#2602) that will allow the user to trigger the x-post UI by typing the "+" character.

Describe alternatives you've considered

The possibility of using a action sheet or bottom sheet instead of a popup menu was also discussed.

[guarani]

This was done in WordPress/gutenberg#26211 (or to be more specific, commit WordPress/gutenberg@e730f2e and WordPress/gutenberg@7e3662a).

These changes were then brought into the WordPress apps via wordpress-mobile/WordPress-iOS#15139 and wordpress-mobile/WordPress-Android#13184.