Main screen: update disabled/hidden tabs state if a model has not been selected

Design coming soon

User will only see the tabs disabled/hidden when they haven't selected a model from a dropdown (but do have the option "Load an existing model" selected - this is also selected by default when user first open the plugin)

Check with @yrkim98 what is the available option: 1. completely have the tabs hidden; or 2. make them dim/look disabled

Split from this ticket https://github.com/AllenCell/allencell-ml-segmenter/issues/279 and interdependent on this ticket https://github.com/AllenCell/allencell-ml-segmenter/issues/260