fix(components): change placeholder color when datepicker has a value

[rmarquois]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Set a different color for placeholder and selected date on datepicker to have the same behavior as the Input. It helps to have a homogeneous form. This issue is related : #1224.

I also added this to my mapping.json :

{
	"components": {
		"Datepicker": {
			"appearances": {
				"default": {
					"variantGroups": {
						"status": {
							"basic": {
								"textColor": "text-basic-color",
								"state": {
									"disabled": {
										"labelColor": "text-disabled-color"
									}
								}
							}
						}
					}
				}
			}
		}
	}
}

[djbuch]

This should be merged !