Major code refactor. Migrate to null-safety.

[MarcinusX]

Changelog:

• Migrate to null-safety.
• Changed named constructors to two separate widgets.
  • NumberPicker.integer is now NumberPicker.
  • NumberPicker.decimal is now DecimalNumberPicker.
  • NumberPicker.horizontal is now NumberPicker with axis: Axis.horizontal
• Removed infiniteLoop to make it possible to go to null safety. The feature will come back once the infitniteListView package gets null-safe.
• Removed dialogs containing pickers. I think it's better to just leave it for developers.
• Removed animateInt method and replaced it with just reacting to value changed (see example).
• Changed animation curve to Curves.easeOutCubic.
• Changed initialValue to value to avoid confusion from my awful naming.
• Removed highlightSelectedValue as it can be obtained by providing selectedTextStyle.
• Fixed some minor bugs.

Closes #89
Closes #43
Closes #26
Closes #94

[coveralls]

Pull Request Test Coverage Report for Build 77

• 115 of 124 (92.74%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+24.9%) to 92.742%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/src/numberpicker.dart	89	98	90.82%

Totals
Change from base Build 69:	24.9%
Covered Lines:	115
Relevant Lines:	124

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💛 - Coveralls

[MarcinusX]

This should also close #94

[nilsreichardt]

This should also close #94

Can add "Closes #94" to the PR description?

And you have to add the word "Closes" for every ticket. Like this:

Closes #89
Closes #43
Closes #26
Closes #94

Otherwise it will not close the issues

[nilsreichardt]

@MarcinusX I can confirm that the example is working 👍 But I couldn't test in our real app, because other dependencies didn't merged yet.

Screen.Recording.2021-03-07.at.11.47.59.mov