FormFields überarbeiten - `_value`-Handling "aufräumen"

[deleonio]

Wie im Konzept evaluiert: https://github.com/public-ui/kolibri/wiki/%C3%9Cbersicht-Input%E2%80%90Komponenten

Komponente	_value	getValue	Default
kol‑button	StencilUnknown	StencilUnknown	null
kol‑button‑link	StencilUnknown	StencilUnknown	null

Komponente	_value	getValue	onInput	onChange	Default	Bemerkungen
kol‑combobox	string	string	string	string	''	Wenn leer, ist es ein leerer String
kol‑input‑checkbox	StencilUnknown	StencilUnknown	StencilUnknown	StencilUnknown	null	Angehakt gibt den Value und und nicht angehakt gibt null zurück.
kol‑input‑radio	StencilUnknown	StencilUnknown	StencilUnknown	StencilUnknown	null	n/a
kol‑select	StencilUnknown | StencilUnknown[]	StencilUnknown | StencilUnknown[]	StencilUnknown | StencilUnknown[]	StencilUnknown | StencilUnknown[]	StencilUnknown | []	n/a
kol‑single‑select	StencilUnknown	StencilUnknown	StencilUnknown	StencilUnknown	null	n/a

Komponente	_value	getValue	onInput	onChange	Default
kol‑input‑number	NumberString | number	NumberString | number | null	NumberString | number | null	NumberString | number | null	null
kol‑input‑range	NumberString | number	NumberString | number | null	NumberString | number | null	NumberString | number | null	null

Komponente	_value	getValue	onInput	onChange	Default
kol‑input‑date	Iso8601 | Date | null	Iso8601 | Date | null	Iso8601 | Date | null	Iso8601 | Date | null	null

Komponente	_value	getValue	onInput	onChange	Default
kol‑input‑email	string	string	string	string	''
kol‑input‑password	string	string	string	string	''
kol‑input‑text	string	string	string	string	''
kol‑textarea	string	string	string	string	''

Komponente	_value	getValue	onInput	onChange	Default	Bemerkungen
kol‑input‑color	string	string	string	string	'#000'	Standardwert #000 wird genommen, wenn leer

Komponente	_value	getValue	onInput	onChange	Default
kol‑input‑file	n/a	FileList	FileList	FileList	FileList