[BITV/A11Y] Nextcloud dialog title needs to be inside and focus goes on it first

[PVince81]

Steps:

1. Open https://nextcloud-vue-components.netlify.app/#/Components/NcModal
2. Scroll to "Modal with more properties" and click the button

What we need to check:

• title must be inside the dialog (change the default?)
• title must be a h2
• dialog's aria-labelledby must point to the h2
• if a field exists, set the tab there directly

=> test with a screen reader, it should already read the title before announcing the field