Improve setting selection

[Sebastian-ubs]

Describe the bug
After selecting an option, the dropdown only contains the selected option

(Storage functionality will only be implemented with #503, I assume that therefore on focus leave the value is reset to initial value)

To Reproduce
Steps to reproduce the behavior:

Action	Result	Screenshot
1. Open "Interface Language" dropdown	All available
2. Select "Spanish"	Only selected option available
3. Mark all. Type "en"	Only matching options are available

So the narrowing down of the drop down is probably to enable searching, but we don't need this, as jumping to entries also works without the typed characters appearing and filtering the list (see https://mui.com/material-ui/react-select/)

Expected behavior
Don't enable typing into the dropdowns, instead always show all selected options
Also the ability to clear text should not be available here

[Sebastian-ubs]

This should later be part of the settings list in the shared settings component