Make all the valued containers subclass ValueWidget

[hanjinliu]

Closes #372 #661

Now this works:

from magicgui.widgets import FileEdit, PushButton
from magicgui.widgets.bases import ValueWidget

assert isinstance(PushButton(), ValueWidget)
assert isinstance(FileEdit(), ValueWidget)

I had to refactor more than I first thought. The major problem was that ValueWidget had ValueWidgetProtocol before, which is not compatible with container-like widgets (e.g. there's no _mgui_get_value for FileEdit). In this PR, I made ValueWidget more abstract: it only implements the basic interface for getting/setting values, thus independent of backends. PrimitiveValueWidget is the class that require backends, which is identical to the former ValueWidget. Besides, I introduced _BaseContainerWidget for ValuedContainerWidget because many methods of ContainerWidget is not publicly needed for value widgets (e.g. we never use insert for FileEdit).

Inheritance map is now like below.

Widget ----------------> ValueWidget --> PrimitiveValueWidget --> Label, LineEdit, Image etc.
   |                        |
   v                        v
_BaseContainerWidget --> ValuedContainerWidget -----------------> FileEdit, RangeEdit, ListEdit etc.
   |
   v
ContainerWidget ------------------------------------------------> Container

Notes:

• EmptyWidget is now a ValuedContainerWidget because it's simply an empty container. Backend implementation of EmptyWidget is no longer needed.
• ValueWidget now inherits ABC because abstractmethod get_value and set_value are needed. This means if one made a Qt widget and magicgui interface like

  class QMyWidget(QBaseValueWidget): ...
  
  class MyWidget(ValueWidget):
      def __init__(self, **kwargs):
          kwargs["widget_type"] = QMyWidget
          super().__init__(**kwargs)

  then it fails with TypeError due to undefined abstract methods.

Please let me know what you think.

[codecov]

Codecov Report

Attention: Patch coverage is 91.62996% with 19 lines in your changes missing coverage. Please review.

Project coverage is 89.02%. Comparing base (058d63c) to head (b55c3a7).

Files with missing lines	Patch %	Lines
src/magicgui/widgets/_concrete.py	83.09%	12 Missing ⚠️
src/magicgui/widgets/bases/_container_widget.py	93.80%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #663      +/-   ##
==========================================
- Coverage   89.16%   89.02%   -0.15%     
==========================================
  Files          39       39              
  Lines        4746     4721      -25     
==========================================
- Hits         4232     4203      -29     
- Misses        514      518       +4     

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[tlambert03]

this looks like a great start @hanjinliu, the one thing I'm not thrilled about is the changing of the meaning of ValueWidget to PrimitiveValueWidget... it's a breaking change to expect everyone to now inherit from PrimitiveValueWidget, and as you can see it breaks your own magic-class tests here. I'd rather use a shared base class like ValueWidgetBase and have it not be the case that isinstance(TupleWidget(), ValueWidget) than to rename all ValueWidgets

[hanjinliu]

That makes sense. Keeping ValueWidget in the same state is definitely safer.
The magic-class tests failed by the isinstance check of ValueWidget but I think this should be fixed on my side (this is due to the update in EmpytWidget).
The recent breaking change in griffe affects the docs test.