Add smart picker to the web ui

[juliusknorr]

In Nextcloud we have a feature called smart picker that is used in a lot of places to insert links or text.

https://github.com/nextcloud/richdocuments/blob/main/src/mixins/pickLink.js#L8-L19 is how the smart picker can be triggered.

We would want to have a button in Euro-Office webapps that then can send a message to the nextcloud app (maybe through post messages that get passed between the iframe and the parent frame).

[rikled]

First version in Euro-Office/eurooffice-nextcloud#2, Euro-Office/sdkjs#3 and #7, needs some testing once we have a nice build process