Fixes in new_key_frame computation and steps default value

[Fifourche]

Hi !

This PR corrects 2 small things :

• the default steps value displayed from the widget was set to 15, whereas the actual default value from Animation is set to 30. I set both to 30.

• the selected key frames while sliding were not the right ones ; I corrected this by computing the frame number corresponding to each key frame and put it in a new keyframe_to_frame attribute of AnimationSliderWidget.

I also added a bit in the relevant docstrings.

I hope this is ok !

[codecov[bot]]

Codecov Report

Merging #49 (54e87a6) into main (fbea93e) will decrease coverage by 0.41%. The diff coverage is 30.00%.

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
- Coverage   62.53%   62.11%   -0.42%     
==========================================
  Files          15       15              
  Lines         710      718       +8     
==========================================
+ Hits          444      446       +2     
- Misses        266      272       +6     

Impacted Files	Coverage Δ
napari_animation/_qt/animation_widget.py	26.01% <0.00%> (-0.44%)	:arrow_down:
napari_animation/utils.py	92.59% <ø> (ø)
napari_animation/_qt/animationslider_widget.py	35.00% <33.33%> (-0.72%)	:arrow_down:
napari_animation/animation.py	79.10% <100.00%> (ø)

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