Allow user to set filename for new diagram before starting CrypPad

Currently when creating a *New* > *New diagrams.net diagram* CryptPad creates a file in the current folder with a name based on the scheme `diagram-.drawio` e.g. `diagram-123456.drawio`.

It would be very much appreciated if the CryptPad integration would ask the user instead for the preferred filename providing a valid default like the Collabora integration with NC does, where I got the screenshot from:


The default is in this case `New document.odt`, but as there is already a file called `New document.odt` in current folder, a counter is added - in this case ` (1)`.