Compress GUI element allows for arbitrary values

[machawk1]

Describe the bug Compression field in mailbagit-gui allows for invalid values beyond the available values in the dropdown.

To Reproduce

1. Run pip install 'mailbagit[pst]'
2. Run mailbagit-gui
3. Click the text "Select Option" under the "compress" heading.
4. Enter arbitrary text.

Expected behavior The expected behavior is that GUI component is not editable text but when clicking "Select Option", the dropdown contents is displayed, allowing for a selection (i.e., tar, zip, tar.gz). Clicking the arrow to the right of the text exhibits this expected behavior.

Screenshots

Environment (please complete the following information):

• GUI or command line?: (both)
• OS: [e.g. iOS] macOS 12.3.1, M1/arm64 (perhaps significant)
• Python version: Python 3.9 (wxPython seems to have limited support for 3.10+)