adding fullscreenNavigationUI option; fix #482

[cevaris]

Summary
On devices with soft UI buttons (Android/Amazon Fire, etc), when in fullscreen mode, some browsers still renders the soft UI buttons at the bottom of the screen, taking away from the "fullscreen" experience.

This PR adds a new Viewer option fullscreenNavigationUI, to modify that soft UI button display behavior per Mozilla web spec.

Fixes issue #482.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:
Before:
After:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome/Brave
• Firefox
• Safari
• Edge
• IE
• Silk

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

I realize the contributing guidelines ask for a reproducible example.
Although, this particular issues requires a particular device with soft UI buttons and is reproducible via master branch.
Hopefully, that explanation is sufficient and this PR will be allowed.
Thanks!

[codecov-commenter]

Codecov Report

Merging #483 (cd313a9) into master (cdd94b4) will decrease coverage by 0.12%.
The diff coverage is 0.00%.

❗ Current head cd313a9 differs from pull request most recent head c066a63. Consider uploading reports for the commit c066a63 to get more accurate results

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
- Coverage   75.93%   75.80%   -0.13%     
==========================================
  Files           8        8              
  Lines        1421     1422       +1     
==========================================
- Hits         1079     1078       -1     
- Misses        342      344       +2     

Impacted Files	Coverage Δ
src/js/others.js	51.42% <0.00%> (-1.00%)	⬇️
src/js/methods.js	88.64% <0.00%> (-0.03%)	⬇️
src/js/render.js	93.96% <0.00%> (ø)

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[fengyuanchen]

Please do not push any automatically generated files, such as any files in the dist folder.

[fengyuanchen]

Your idea is good, but your solution may be a bit one-sided.