Fix bug #93 Add an isLoading state

[mikeand]

Default value will be true, then set to false after loading complete.

The reason it flickers the No Lists picture is because when initially started the app has no shopping lists loaded. It has no way of telling if this is because there are no lists or the lists are currently being fetched. I add a isLoading state that defaults to true and is set to false when the data is retrieved.

When in an isLoading state, it shows the same color as the splash screen and a spinner. The theme hasn't been loaded at this stage so it's not possible to show the dark or light theme background.

[codecov[bot]]

Codecov Report

Merging #102 (f67b06d) into master (5af8fc8) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #102   +/-   ##
=======================================
  Coverage   76.96%   76.96%           
=======================================
  Files          20       20           
  Lines         534      534           
=======================================
  Hits          411      411           
  Misses        123      123           

Flag	Coverage Δ
app	65.99% <ø> (ø)
bloc	97.32% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...opping_bloc/lib/src/bloc/shopping_lists_cubit.dart	100.00% <ø> (ø)

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