UICoordinator refactor

[WilliamDenniss]

Following on from #196.

[codecov-io]

Codecov Report

Merging #212 into master will increase coverage by 0.51%.
The diff coverage is 12.3%.

@@            Coverage Diff             @@
##           master     #212      +/-   ##
==========================================
+ Coverage   66.51%   67.03%   +0.51%     
==========================================
  Files          41       41              
  Lines        3993     4004      +11     
  Branches       73       73              
==========================================
+ Hits         2656     2684      +28     
+ Misses       1335     1318      -17     
  Partials        2        2

Impacted Files	Coverage Δ
Source/iOS/OIDAuthorizationService+IOS.m	0% <0%> (ø)	⬆️
Source/macOS/OIDAuthState+Mac.m	0% <0%> (ø)	⬆️
Source/macOS/OIDAuthorizationService+Mac.m	0% <0%> (ø)	⬆️
Source/OIDAuthorizationService.m	16.83% <0%> (-0.12%)	⬇️
Source/macOS/OIDRedirectHTTPHandler.m	0% <0%> (ø)	⬆️
Source/iOS/OIDAuthState+IOS.m	0% <0%> (ø)	⬆️
Source/macOS/OIDExternalUserAgentMac.m	0% <0%> (ø)
Source/OIDAuthState.m	42.27% <0%> (ø)	⬆️
Source/iOS/OIDExternalUserAgentIOS.m	0% <0%> (ø)
Source/iOS/OIDExternalUserAgentIOSCustomBrowser.m	0% <0%> (ø)
... and 3 more

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