TODOS

Add support for opening multiple files
- array contains filenames
- next btn switches to next preview.
Use threshold_display label as messagebox for showing
- open instruction
- threshold val
- saved status and filepath
Update window title with opened filename
Show stacked previews instead of side-by-side view
Make self.threshold_display relative to content size or window size
Make previews resize on window resize
Use low-quality image for perviews
[-] Support opening of any image format.
Fix previews and threshold display not updating when next or prev btns are clicked.
Rearrange imports in alphabetical order.
(Find a way to) Add ImageMagick to app bundle
Disable prev and next btns when the curr_idx is first and last, respectively.
Dont show saved msg when none of the file is saved.
add support for showing total no. of images loaded and the curr image's no. like '3 of 34'.

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