Add unified dragging API to UI elements

[Nibba2018]

Replaces boiler plate code in various UI elements to make them draggable. Solves #401 .

Any new UI component can use self.set_draggable_components method from its UI super class to implement dragging without any additional code.

[codecov[bot]]

Codecov Report

Merging #635 (f385250) into master (8e1cc27) will increase coverage by 0.01%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #635      +/-   ##
==========================================
+ Coverage   50.19%   50.20%   +0.01%     
==========================================
  Files         120      120              
  Lines       28019    27965      -54     
  Branches     2987     2981       -6     
==========================================
- Hits        14063    14039      -24     
+ Misses      13489    13461      -28     
+ Partials      467      465       -2     

Impacted Files	Coverage Δ
fury/ui/containers.py	0.00% <0.00%> (ø)
fury/ui/core.py	0.00% <0.00%> (ø)
fury/ui/elements.py	0.00% <0.00%> (ø)
fury/ui/tests/test_helpers.py	0.00% <ø> (ø)
fury/fury/ui/core.py	92.97% <0.00%> (-0.42%)	:arrow_down:
fury/fury/ui/containers.py	83.05% <0.00%> (-0.32%)	:arrow_down:
fury/fury/ui/elements.py	89.65% <0.00%> (-0.02%)	:arrow_down:
fury/fury/ui/tests/test_helpers.py	100.00% <0.00%> (ø)
fury/fury/stream/tools.py	91.15% <0.00%> (+0.21%)	:arrow_up: