Button and Slider Widgets

[amalss18]

This will allow the user to add a slider, button to the scene and associate callbacks with these widgets.

[codecov-io]

Codecov Report

Merging #844 into master will decrease coverage by 0.09%. The diff coverage is 39%.

@@            Coverage Diff            @@
##           master     #844     +/-   ##
=========================================
- Coverage   50.85%   50.76%   -0.1%     
=========================================
  Files         263      265      +2     
  Lines       23846    24046    +200     
  Branches     3252     3270     +18     
=========================================
+ Hits        12128    12206     +78     
- Misses      10931    11053    +122     
  Partials      787      787

Impacted Files	Coverage Δ
mayavi/mlab.py	81.25% <100%> (+1.25%)	:arrow_up:
mayavi/sources/widget_source.py	27.58% <27.58%> (ø)
mayavi/tools/widgets.py	40.58% <40.58%> (ø)

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