WIP:add a new widget called ReadbackButton.

[lvhuihui1988]

…on in EDM that has two PV properties. One is Control PV and the other is Readback PV.
In edm, the button widget has two PV properties. One is Control PV, and the other is Readback PV. We can also set the color in Inconsistent property. When the two PVs' value are different, the inconsistent color will shown on the button.
Similarly, we need this functionality in PyDM GUIs. So I write a new widget called ReadbackButton. In this widget, I add a readback channel and inconsistentColor properties, as well as corresponding slot functions, relative to the PushButton widget.

[lvhuihui1988]

In edm, the button widget has two PV properties. One is Control PV, and the other is Readback PV. We can also set the color in Inconsistent property. When the two PVs' value are different, the inconsistent color will shown on the button.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 272 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.43%. Comparing base (9ac9bd5) to head (97eff3e).
⚠️ Report is 1708 commits behind head on master.

Files with missing lines	Patch %	Lines
pydm/widgets/readbackbutton.py	0.00%	270 Missing ⚠️
pydm/widgets/qtplugins.py	0.00%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #774      +/-   ##
==========================================
- Coverage   58.92%   57.43%   -1.49%     
==========================================
  Files          91       92       +1     
  Lines       10543    10815     +272     
==========================================
  Hits         6212     6212              
- Misses       4331     4603     +272     

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