Refactored validated input widgets to be more extensible

[alexhroom]

This PR refactors validated input widgets to a more object-oriented system that makes them easier to extend and change. Fixes #41.

To add a new input widget, the user should create a subclass of BaseInputWidget which has one method and three class attributes defined:

class MyInputWidget(BaseInputWidget):
    """My special input widget."""
    
    data_getter: str = "myGetter"  # data getter method for the editor
    data_setter: str = "mySetter"  # data setter method for the editor
    edit_signal: str = "editedSignal"  # method for signal editor produces when changed

    def create_editor(self, field_info: FieldInfo) -> QtWidgets.QWidget:
        """Method that creates the editor widget from Pydantic field info."""
        # whatever goes here...

and then register the widget and its relevant type in the class_widgets dictionary in a new get_validated_input function. This will produce an input widget which has methods editor for its editor, get_data for its data getter, set_data for its data setter and edited_signal for its change signal as before.

Accessing the validated input widgets is almost the same: we just change widget = ValidatedInputWidget(field_info) to widget = get_validated_input(field_info).

This means that the creation of editors is more encapsulated for each type, rather than the previous system of "just get the type of widget from the data type and then use this big if/elif/elif... block to sort out the relevant validation"

[StephenNneji]

Looks good to me, please see comment

[StephenNneji]

The names type and input shadow built-in globals while I don't think anyone will every use input in this project, I advise changing these names to avoid conflict or bugs in future. Might be good to add this to our linter if its easy to do

[alexhroom]

good spot - i've added the flake8-naming ruleset to the linting and fixed a couple other places where it was broken.