Expose more options in UI and new UI features

[nuest]

Extending #27

Relevant to all dialogs:

• checkbox for versions
• maintainer text field
• image
• Change the "Done" text to "Create" or something else
• ...

New features:

• add a "Do you want to overwrite" check before writing a Dockerfile that already exists
• add UI for packaging DESCRIPTION files, potentially with a drop-down of installed packages
• ...