Load Notifications and Emoji Engine on Demand

[caronc]

Description:

Related issue (if applicable): n/a

This is a massive re-factor of how notifications are loaded into memory. It also refactors the way the Emoji engine is loaded (saving on memory when it's not needed).

After this PR, notifications are no longer loaded on the import apprise command. Instead leveraging a new library, they are only imported on demand.

• Improved startup logging and information on load times (both Native Notifications and Custom ones loaded thereafter)
• Testing re-factored to better handle the dynamic reloading of modules to test how they handle.

Breaking Changes

• This will require the Apprise API to be refactored to work correctly once this is merged. It will also break anyone who learned of the 'under-the-hood' Apprise environment and was manipulating the old SCHEMA -> Module mapping.

Those using the library as it was intended will not see any breakage at all.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cda6925) 99.55% compared to head (b421dfd) 99.27%.
Report is 2 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1020      +/-   ##
==========================================
- Coverage   99.55%   99.27%   -0.28%     
==========================================
  Files         130      136       +6     
  Lines       17281    17657     +376     
  Branches     3523     3603      +80     
==========================================
+ Hits        17204    17529     +325     
- Misses         68      119      +51     
  Partials        9        9              

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