Standardize full screen mode for viewer integrations

[juliushaertl]

Collabora and Text use a bit of a hacky approach to make the viewer show their content full screen with the special case that for Collabora the viewer header is actually hidden and the close/sidebar buttons are actually rendered by Collabora.

With #1229 that basically caused some regressions on the Text and Collabora side. To avoid that it would be good to have some indicator on the apps viewer integration that the modal content should span full width and height.

This could be handled similarly to #1051 to be an additional option on the handler registration.

Any objections @skjnldsv ?

[skjnldsv]

Collabora and Text use a bit of a hacky approach to make the viewer show their content full screen with the special case that for Collabora the viewer header is actually hidden and the close/sidebar buttons are actually rendered by Collabora.

With #1229 that basically caused some regressions on the Text and Collabora side. To avoid that it would be good to have some indicator on the apps viewer integration that the modal content should span full width and height.

This could be handled similarly to #1051 to be an additional option on the handler registration.

Any objections @skjnldsv ?

None, pdf viewer would also love this :)