SlidePane and Dialog updates and conversion to Classes

[tomdye]

Type: bug / feature

The following has been addressed in the PR:

• All code matches the style guide
• Unit or Functional tests are included in the PR

Description:

• Make use of null classes functionality
• Drop -attr usage
• Add Enum for Align
• Add const for min-width
• renamed to slidePane
• dropped camelCase from folder name

Converts both slidepane and dialog to use Classes

Requires

• Widget-core release
• Conversion to Classes

[codecov-io]

Codecov Report

Merging #28 into master will decrease coverage by -0.95%.
The diff coverage is 99.05%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage     100%   99.05%   -0.95%     
==========================================
  Files           3        3              
  Lines         105      106       +1     
  Branches       44       46       +2     
==========================================
  Hits          105      105              
- Partials        0        1       +1

Impacted Files	Coverage Δ
src/main.ts	100% <100%> (ø)	✅
src/dialog/Dialog.ts	100% <100%> (ø)
src/slidepane/SlidePane.ts	98.66% <98.66%> (ø)

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[tomdye]

Apologies to all for the diffing of createSlidePane.ts, GH doesn't seem to be happy with it being changed, renamed and moved into a new directory.

[tomdye]

@bitpshr have pulled your changes into this PR.

[bitpshr]

@tomdye I think we need to bump the widget-core version so the CI will pass, then hopefully we can land this.

[bitpshr]

Not sure why this one line isn't covered despite being hit 15 times: https://codecov.io/gh/dojo/widgets/compare/023ecfe575368ecbdfbabf6163f0f77545362085...c28cf622ef916b5ec451f16a46c7e94c6661bf18/diff#D19-47.