Button / select box - font size attribute

[AndiLeni]

Describe the bug
font_size attribute is not working for buttons and select boxes.

To Reproduce

button = toga.Button(
            'Eintrag speichern',
            on_press=self.say_hello,
            style=Pack(padding=5, font_size=15)
        )

or
toga.Selection(items=['m', 'w'], style=Pack(font_size=15))

Expected behavior
The font size should change.

Screenshots
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Environment:

• Operating System: windows 10
• Python version: 3.8.5
• Software versions:
  • Briefcase: 0.3.3
  • toga-core==0.3.0.dev22
  • toga-winforms==0.3.0.dev22

Additional context
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[shape-warrior-t]

I've been looking into this issue. As of now, on Windows, font_size works for buttons, but not select boxes. The same thing also happens to be true for Android, based on my testing.

I'll try and see if there's anything I can do to quickly fix this bug.

EDIT: Taking a quick look at the source code for other platforms, I can't identify any functions for setting fonts on any of them. So I suspect that select boxes simply don't support setting the font as of right now.

[freakboy3742]

#1761 Addressed this issue, and added runtime tests of this behavior.