Refactor FileInputWidget and Viewer

**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.