Add PHP API for registering custom gradient presents

[spacedmonkey]

As originally discussed in #17169 (comment) and with work continue #17603 .

In #17169 gradient presents were added to buttons. This function similar to the present colors already found in gutenberg. The present colours allow for register of list of colours in PHP using theme supports. Example.

add_theme_support( 'editor-color-palette',
			array(
				array(
					'name'  => __( 'Primary', 'twentynineteen' ),
					'slug'  => 'primary',
					'color' => twentynineteen_hsl_hex( 'default' === get_theme_mod( 'primary_color' ) ? 199 : get_theme_mod( 'primary_color_hue', 199 ), 100, 33 ),
				),
				array(
					'name'  => __( 'Secondary', 'twentynineteen' ),
					'slug'  => 'secondary',
					'color' => twentynineteen_hsl_hex( 'default' === get_theme_mod( 'primary_color' ) ? 199 : get_theme_mod( 'primary_color_hue', 199 ), 100, 23 ),
				),
				array(
					'name'  => __( 'Dark Gray', 'twentynineteen' ),
					'slug'  => 'dark-gray',
					'color' => '#111',
				),
				array(
					'name'  => __( 'Light Gray', 'twentynineteen' ),
					'slug'  => 'light-gray',
					'color' => '#767676',
				),
				array(
					'name'  => __( 'White', 'twentynineteen' ),
					'slug'  => 'white',
					'color' => '#FFF',
				),
			)
);

A similar api should be added for gradient presents. I would recommend the name editor-gradient-presets.

[spacedmonkey]

I have a WIP PR #17841

While working on this PR, I am starting to believe that another theme_supports should be added called disableCustomGradients to bring it inline with colors.

[spacedmonkey]

I think we should add add_theme_support('disable-gradient-presets'). See #18213

[bbertucc]

@spacedmonkey - I'm working on a site now with brand guidelines that specifically discourage using gradients, so yes- would love add_theme_support('disable-gradient-presets')

[spacedmonkey]

@bbertucc Have you seen https://github.com/WordPress/gutenberg/pull/17603/files#diff-bda1f53deac273d361a56c63a5b90adeR146 ?

[bbertucc]

Thanks @spacedmonkey - That's an awesome experimental fix. Should I be worried about the __experimental part of the variable? Once its mature, will there be a fallback, or should I just keep tabs on #18213 for updates?