Cleanups of testbed probe usage to get some missing coverage.

[freakboy3742]

The recent audit of Divider missed some coverage because a probe was being used instead of a native getter on the implementation. The audit of Box somehow completely missed committing the entire GTK and Winforms implementations.

This PR:

• updates the tests to use the native widget getter rather than a probe property when a widget getter exists.
• Adds the Winforms and GTK implementation of the Box probe that was somehow missed in the commit of [widget audit] toga.Box #1820.
• Reworks the way GTK applies style to ensure that the native widget will always have the required CSS style attributes.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

• Adds the Winforms and GTK implementation of the Box probe that was somehow missed in the commit of [widget audit] toga.Box #1820.

I think the implicit skip approach causes too much risk of tests (and therefore bugs) being missed, so I've changed it to explicitly skip tests for widgets which are known to be unimplemented on the current platform. I prefer this over listing the platforms which *are" implemented, because it's safer to default to running too many tests rather than too few.

[freakboy3742]

Good call - explicit skipping will definitely be less prone to error (and won't hide misspelled probe/missing problems).