Fix #681 Add 'TestFontMetrics' class to approximate real font width calculations

[dhauge]

• Add class TestFontMetrics that implements a get_font_metrics method that has configurable widths that can vary by character
• Add function calculate_font_metric_info_for_tests that can produce a python code snippet that constructs a TestFontMetrics instance for a specified font, using the Qt library
• Add function make_font_metrics_for_tests that constructs a TestFontMetrics instance for use by automated tests, using the code snippet returned by calculate_font_metric_info_for_tests called from a script console for font "normal 11px serif"
• Modify TestUI.UserInterface.get_font_metrics to calculate font metrics using the TestFontMetrics returned by make_font_metrics_for_tests

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[dhauge]

Are type declarations required for this project? I've gotten several build failures because types were missing or the type of a variable changes:

nion/ui/TestUI.py:38: error: Need type annotation for "font_width_by_char" (hint: "font_width_by_char: Dict[<type>, <type>] = ...")

Is the intent to have everything in the project strictly typed?

[cmeyer]

Is the intent to have everything in the project strictly typed?

The intent (for now) is to pass the mypy type checking, and there are still spots where type checking is missing. There is a more strict version of typing where everything is declared to have been typed. See this page if you're interested. I don't this this package is to that level yet.