Fix proof task tree forced white background.

[wadoon]

Related Issue and Solution

The proof task tree (left upper component in the UI) has a forced white background if it is empty initially. This does not respect LaFs.

To be more precise, the logic was as follows:

1. The scroll pane was empty initally (view port not set)
2. The default view port was requested and the background was set to white.
3. If finally a proof was loaded, the view port was set to the appropriate JList.

New logic:

1. Just set the JList directly.

The rest is clean-up of unused methods.

Type of pull request

• Bug fix (non-breaking change which fixes an issue)

• There are changes to the (Java) code

Ensuring quality

• Visual inspection in Dark Theme and Metal. Not coverable by automatic tests.