Add apple_touch_startup_images filter

[jefferyto]

This adds an apple_touch_startup_images filter for plugins/themes to adjust the rel="apple-touch-startup-image" link tags outputted. This allows more images to be added, with an applicable media attribute for each image.

Fixes #586.

[jefferyto]

The test failure for "Unit test: PHP 8.0, WP trunk" appears unrelated to this change.

[westonruter]

The test failure for "Unit test: PHP 8.0, WP trunk" appears unrelated to this change.

Yeah. That's something I need to fix along the lines of what @pierlon has done in ampproject/amp-wp#6525.

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[westonruter]

@googlebot I consent.

[codecov-commenter]

Codecov Report

Merging #595 (50e7daf) into develop (4aa2b8a) will increase coverage by 0.54%.
The diff coverage is 92.30%.

@@              Coverage Diff              @@
##             develop     #595      +/-   ##
=============================================
+ Coverage      17.51%   18.05%   +0.54%     
- Complexity       300      305       +5     
=============================================
  Files             55       55              
  Lines           1776     1789      +13     
=============================================
+ Hits             311      323      +12     
- Misses          1465     1466       +1     

Flag	Coverage Δ
php	18.05% <92.30%> (+0.54%)	⬆️
unit	18.05% <92.30%> (+0.54%)	⬆️

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Impacted Files	Coverage Δ
wp-includes/class-wp-web-app-manifest.php	58.73% <92.30%> (+3.86%)	⬆️

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