Allow prioritizing default file actions for a mime type (per user)

Steps to reproduce

1. Enable richdocuments to have rich editing for odt files
2. Enable files_texteditor because you want txt files to be simple
3. Open an txt file: richdocuments starts

I think it would be nice to have something like a priority of the actions. Currently the last one wins, and since richdocuments is after files_texteditor in the alphabet, files_texteditor has no way to ever win this.

Proposal is to add something like priority to: https://github.com/nextcloud/server/blob/2b794902014264a3793daf42a2ecfa95a1a07342/apps/files/js/fileactions.js#L144-L144

and then:

			if (!!this.actions[mime][name] && this.actions[mime][name].priority < actionSpec.priority) {
				return;
			}

This would allow the files_texteditor to register for text files, while richdocuments can also, so in case files_texteditor is not there, you can still edit them.

Alternate approach would be hacking into richdocuments to only register, when files_texteditor is not there.

cc @MorrisJobke @LukasReschke

cc @RealKelsar for reporting it in IRC

Start at apps/files/js/fileactions.js around line 144 and inspect how richdocuments and files_texteditor register actions for text and ODT MIME types. Trace the current last-registration-wins behavior, then verify that an explicitly prioritized action wins while the lower-priority action remains available when the higher-priority app is absent.