Refactoring & improvements

[Feynallein]

Hello,
I worked a lot on your widget in order to improve it:

• Reindenting code
• Making the example clearer
• Improved the angle to hour conversion
• Improved the number's placements on the clock
• Made the whole widget more adaptable at different sizes
• Adding comments to widget's parameters
• Changed project's tree file so it's easier to locate stuff
• Renaming some functions/variables to make them clearer
• Changed the callback's return value to a new type SimpleTime (which holds two variables: hours & minutes) + a bunch of toString methods (toString, toDuration, toAMPMString) and few constructors.

Feel free to ask me any question if needed!
I hope that you will accept this pull request.

Have a good day.

[Feynallein]

Edit:

• Added initial values option
• Removing Flutter 3.0 warning (with WidgetsBinding)
• Fixed a bug with clock's display