Improve the dialog design

[mxsph]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
The dialog (e.g. to create a conference) does not yet fit to the rest of the design, so this should be adjusted.

Furthermore, we should also consider some dialogs again whether we really need them and can not better replace by Popover or other things.

Describe alternatives you've considered
Do not adjust the dialogs.

Additional context
Follow up #55