Custom element updates for dynamic loading

[maier49]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:
Framework changes as described in #770

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[codecov]

Codecov Report

Merging #825 into master will decrease coverage by 2.81%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #825      +/-   ##
==========================================
- Coverage   97.64%   94.83%   -2.82%     
==========================================
  Files         127      127              
  Lines        8034     8034              
  Branches     1861     1862       +1     
==========================================
- Hits         7845     7619     -226     
- Misses        189      415     +226     

Impacted Files	Coverage Δ
src/core/registerCustomElement.ts	6.53% <50.00%> (-91.03%)	⬇️
src/core/mixins/Themed.ts	99.00% <0.00%> (-1.00%)	⬇️
src/core/vdom.ts	97.91% <0.00%> (-0.16%)	⬇️

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[rorticus]

Do you think its worth making the promise bit optional for backwards compatibility reasons? So we basically look and see if a promise was returned and if so, await it, otherwise just carry on?

[maier49]

@rorticus Awaiting a synchronous value is fine though right? And the _readyCallback is already called asynchronously so I'm not sure where the problem would be there. The issues with backwards compatibility would be always expecting a factory function instead of the WidgetConstructor itself like we used to. But I'm not sure there's a way around that (and it should be fixed by just rebuilding unless it was being used manually).