Changes to nnUNet after hackathon

Summary:
This diff includes changes for T29061, T29064, T29065, T29066, T29068 and code cleanup.

Results_folder parsing order changed. Task will be first selection from on instead of Model. Also, Results_folder path will be now searched in system environment too before fetching the MITK stored value from last sessions.

Required input modalities will be auto displayed on UI depending on the selected task.

Python env selection is now paramount for the working of the nnUNet GUI.

MultiModal toggle selection deactivated if mitk json file is found in the Results_Folder. Required Modalities will be fixed as per the values in json file.

Preview object selection & confirm segmentation behaviour is now consistent with other tools.

Vestigial Stop button code cleanup.

Remove No-Pip option & code cleanup.

Intro text update to indicate that labels will be merged while Confirm.

Test Plan: Checklist update pending

Reviewers: kislinsk, kalali, O1 MITK Reviewer Group I

Reviewed By: kalali, O1 MITK Reviewer Group I

Subscribers: floca

Differential Revision: https://phabricator.mitk.org/D627