Refactor FileInputWidget and Viewer

[yrkim98]

Please describe the problem that needs to be solved by this feature
FileInputWidget relies heavily on the viewer for file paths which works but makes these two very tightly coupled.
The viewer is needed for file state when images have been dragged and dropped into the napari viewer or loaded in through napari
We should look into incorporating both as a single component, or refactoring their responsibilities so the current setup relies less on the viewer.

Proposed changes: combine FileInputWidget and Viewer

How important is this feature to you? (Software team will re-prioritize all tickets based on current workload)
medium

Describe the solution you'd like
combine FileInputWidget and Viewer

Describe alternatives you've considered
refactor so FileInputWidget does not rely on Viewer as much.

[yrkim98]

idea of having ViewerService to make changes to viewer based on model state