* Rename customizer JS files to customize-preview.js and customize-controls.js to align with the core file naming and make it clearer where each file runs.
* Only show the colorscheme_hue control when there's a custom color scheme.
* Update preview JS handling for revised front page section handling, see below.
* Remove all references to "Theme Customizer" in code comments. It hasn't been called that since before 4.0.
* Clarify the purpose of the JS files by updated the code comments in the file headers.
* Improve code readability.
* Make the arbitrary number of front page sections filterable, for UI registration and output.
* Rename twentyseventeen_sanitize_layout to twentyseventeen_sanitize_page_layout to be clearer about what it sanitizes in case child themes or plugins consider reusing it.
* Rename page_options setting/control to page_layout as that's more reflective of what that option does; and again, helps for potential extensions.
* Make the page layout option contextual to pages and the sidebar being inactive, as the option only applies when there is no sidebar (per its description).
* Condense options into a single section.
* Add selective refresh for front page sections.
* Locate active_callback functions within customizer.php so that they're easier to find when editing customizer registrations, similarly to sanitize callbacks.
* Adjust the styling for placeholders for panels that aren't active.
* Ensure that the new visible edit shortcuts don't have any issues.
Props celloexpressions.
Fixes#38426.
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git-svn-id: http://core.svn.wordpress.org/trunk@38929 1a063a9b-81f0-0310-95a4-ce76da25c4cd
