(modal) dialogs: keyboard navigation

[marcusm7]

• As mentioned in Keyboard navigation is not confined to open modals #65: the tab leaves the dialog and walks through any element of the table.
• There should be a way to navigate from one element (text field, radio button,...) to the next in any dialog. I.e. by means of the cursor keys or a sane tab-cycle (which then cycles through all elements of the dialog in order).
  In the create event dialog, this works really nice in the dialog's tab-bar and partially in the source-dialog, but i.e. not in the metadata - I can't reach series or presenter by means of the keyboard.
• ESC: Hitting Esc loses all the entered data without a chance to overthink your decision. Although I know this... behaviour, I accidentally close the dialog from time to time. The decision to do it this way is not at all common in other web-UIs. A hint like "Esc again to omit all entered data" or so would be great.

related issues: #607 #65

All tested with:
$ firefox --version
Mozilla Firefox 131.0.2