Add ProgressBar widget

[tlambert03]

Just a basic progress bar widget, as a start to working on #25 ... which will come in another PR

[codecov]

Codecov Report

Merging #104 (8759723) into master (699b9a4) will increase coverage by 0.03%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
+ Coverage   86.03%   86.06%   +0.03%     
==========================================
  Files          14       14              
  Lines        1740     1751      +11     
==========================================
+ Hits         1497     1507      +10     
- Misses        243      244       +1     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/__init__.py	100.00% <ø> (ø)
magicgui/widgets/__init__.py	100.00% <ø> (ø)
magicgui/backends/_qtpy/widgets.py	82.10% <90.90%> (+0.22%)	⬆️
magicgui/widgets/_concrete.py	81.85% <100.00%> (+0.15%)	⬆️

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