Add mouse coordinates display in live viewer

[codejaeger]

PR Checklist

If you are contributing to Javis.jl, please make sure you are able to check off each item on this list:

• Did I update CHANGELOG.md with whatever changes/features I added with this PR?
• Did I make sure to only change the part of the file where I introduced a new change/feature?
• Did I cover all corner cases to be close to 100% test coverage (if applicable)?
• Did I properly add Javis dependencies to the Project.toml + set an upper bound of the dependency (if applicable)?
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Demo

Link to relevant issue(s)
Closes #177

How did you address these issues with this PR? What methods did you use?
Added the mouse coordinates into a Gtklabel. There were 2 ways to do this.

1. Use a Gtk.EventBox (then add the canvas to it) and enable the POINTER_MOTION event in the event box. Then connect a callback to update the coordinates.
2. Make use of the property motion from the Gtk.Canvas and add a callback for the update.

References:-

1. https://stackoverflow.com/questions/58683788/gtk-how-to-detect-the-mouse-position-over-eventbox
2. https://juliagraphics.github.io/Gtk.jl/latest/manual/canvas/

[codecov]

Codecov Report

Merging #326 (7120fa5) into master (4241ea1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #326   +/-   ##
=======================================
  Coverage   96.17%   96.18%           
=======================================
  Files          21       21           
  Lines        1151     1154    +3     
=======================================
+ Hits         1107     1110    +3     
  Misses         44       44           

Impacted Files	Coverage Δ
src/javis_viewer.jl	98.24% <100.00%> (+0.09%)	⬆️

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[Wikunia]

Looks great!
Two things:

• Please round the pixel values otherwise it screws up the slider and buttons a bit when moving around with the mouse.
• I think users are more interested in the coordinates where the origin (center of the image) is 0, 0

[codejaeger]

@Wikunia thanks for the review, I made the changes.

[Wikunia]

Thanks we had a look at this yesterday and we would like to make it a bit clearer with x= ..., y = ... and maybe put it under the frame functionality or smaller and to the right instead of centered.
Anything else we wanted @TheCedarPrince ?