Enable decorator to be used on methods

[tlambert03]

closes #53 by enabling @magicgui decorator on class methods:

class MyObject:
    def __init__(self):
        self.counter = 0

    @magicgui(auto_call=True)
    def process_image(self, sigma: float = 5):
        self.counter = self.counter + sigma
        print(self)

m = MyObject()
m.process_image  # This a FunctionGui with `m` bound to `self` when calling the method.

(see also: examples/class_method.py )

This works by adding a __get__ method to the FunctionGui class, making it a descriptor that will pass the instance from which it is called as the first argument when calling the method.

[codecov]

Codecov Report

Merging #56 (671b3fe) into master (008b0c7) will increase coverage by 0.08%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   77.57%   77.66%   +0.08%     
==========================================
  Files          16       16              
  Lines        1793     1831      +38     
==========================================
+ Hits         1391     1422      +31     
- Misses        402      409       +7     

Impacted Files	Coverage Δ
magicgui/function_gui.py	81.30% <85.71%> (+0.86%)	⬆️
magicgui/widgets/_bases.py	77.99% <85.71%> (-0.12%)	⬇️

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