Checkboxes in Widget

[newhinton]

Fixes #1907 Requires: https://github.com/stefan-niedermann/nextcloud-commons/pull/215

This implements checkable-checkboxes on devices with api 31 or greater.

This is a first working prototype. It can:

• Display a checkbox for chechbox items
• Handle the state correctly

It cannot:

• Display checkbox-text nice. The raw-markdown is currently not stripped
• Send a checkbox event back to the app
• Render the notes good. (With each markdown block, there is a margin at the bottom)

Since this requires changes in the nextcloud-common library, i would like to ask for a general review now, before i continue.

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[AndyScherzinger]

Rebased due to major technical updates in terms of libraries, especially AGP and SSO