Adding Custom Icon in Elementor's default icons array. This snippet created by Asif Islam from iThemesLab.

functions.php

<?php

function itl_icons_filters( $controls_registry ) {

	// Get existing icons
	$icons = $controls_registry->get_control( 'icon' )->get_settings( 'options' );

	//Create array with your icon set
	$newIcons = array(
		'icon-001-worldwide' => 'worldwide',
		'icon-008-like' => 'like',
		'icon-025-globe' => 'globe',
		'icon-017-mobile' => 'mobile',
	);

	//Inject new icon set into default icon list array
	if ( !empty($newIcons) && is_array( $newIcons ) ) {
		$icons = array_merge( $newIcons, $icons);
	}

	// send back new array
	$controls_registry->get_control( 'icon' )->set_settings( 'options', $icons );
}
add_action( 'elementor/controls/controls_registered', 'itl_icons_filters', 10, 1);

// enqueue css file into editor
function itl_new_icon(){
    wp_enqueue_style( 'new-icon', plugins_url( 'YOUR_ICON_CSS_PATH',  __FILE__ )  );
}
add_action( 'elementor/editor/after_enqueue_styles', 'itl_new_icon' );

//enqueue css file for front-end
function seokit_core_assets() {
    wp_enqueue_style( 'new-icon', plugins_url( 'YOUR_ICON_CSS_PATH',  __FILE__ )  );
}
add_action( 'wp_enqueue_scripts', 'seokit_core_assets' );