Upload model checkpoint

[ricdodds]

Add the ability for users to upload pre-trained models from the ImPartial plugin UI in ImageJ. Assumptions

• There must have uploaded at least one image to the dataset first
• Is responsibility of the user to ensure the number of channels of the model match the number of channels of the dataset
• The pre-trained model can have arbitrary filename, so that users can use meaningful names to store their models
• Introduce the BackgroundTaskRunner to standardize the way long tasks are ran in the background without blocking the UI

[gunjan-sh]

Need to be reviewed after the merge.