Fix excessive TMDB requests sent when viewing movie details

[nivanchikov]

Issue: The movies list gets re-fetched each time the user opens or exits the movie details screen on a device with a compact-width trait (e.g. iPhoneX or iPhone XS Max in portrait orientation). See the attached gif with a dump of requests being sent when I was opening various movie details

Why this happens:

• the searchBar.rx.text event stream triggers the movie search state updates and TMDB requests
• the events for the text control property are triggered by both text editing events and responder events (e.g. begin/end editing)
• when the user opens the movie details screen, the search bar resigns the responder thus triggering the main state update and dispatching a new TMDB request

Proposed solution: This pull request adds a couple of new actions to differentiate when the movie details screen is always visible in a split view controller. If the details screen is currently the only visible one, e.g. in a compact-width device orientation, the searchBar.rx.text events are additionally filtered to disallow updating the search state when the search screen is not visible

[codecov-io]

Codecov Report

Merging #35 into master will increase coverage by 0.34%. The diff coverage is 83.33%.

@@            Coverage Diff            @@
##           master     #35      +/-   ##
=========================================
+ Coverage   69.85%   70.2%   +0.34%     
=========================================
  Files          13      13              
  Lines         428     443      +15     
=========================================
+ Hits          299     311      +12     
- Misses        129     132       +3

Impacted Files	Coverage Δ
ReduxMovieDB/State/MainState.swift	100% <100%> (ø)	:arrow_up:
...xMovieDB/Controllers/MovieListViewController.swift	89.58% <100%> (ø)	:arrow_up:
ReduxMovieDB/Controllers/SplitViewController.swift	48% <25%> (-4.39%)	:arrow_down:

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[cardoso]

Hi @nivanchikov 👋 thanks for your contribution! I'll review as soon as possible.