Fix #3009 (iOS default widget background color)

[proneon267]

A bug has slipped past me in #3009. The current implementation sets backgroundColor = None as the default background color. The docs also mentions that None or nil gets the default background color, which is the transparent background color: https://developer.apple.com/documentation/uikit/uiview/backgroundcolor?language=objc:

Changes to this property can be animated. The default value is nil, which results in a transparent background color.

However, contrary to the docs, when backgroundColor = None is set as the default background color for UILabel, then it displays a black background color instead:

On further testing, I have found that the default background color(default value of native.backgroundColor) which the system assigns to UILabel on initialization is not None/nil or even UIColor.clearColor, but rather UIExtendedGrayColorSpace 0 0.

I couldn't find any official docs for UIExtendedGrayColorSpace 0 0, the best I could get was: https://stackoverflow.com/a/45491411, which suggests that:

UIExtendedGrayColorSpace 0 0 is "clear"

So, it seems contrary to the docs, setting backgroundColor to nil doesn't always set it to the widget's system assigned default background color. So, the best option is to cache the system assigned widget default background color during the widget initialization and use that as the default reset background color, in order to avoid any similar background color bugs.

I have tested this PR with both light mode and dark mode, and it works correctly even when the system theme is changed while the app is running:

Light Mode to Dark Mode
Dark Mode to Light Mode

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

[freakboy3742]

Nice catch.

I'd feel a lot better if there was a way to automate testing of this, but I appreciate that this is one of those areas where the property we're trying to test is inherently visual.