Add helper text slot and API

[juuso-vaadin]

Add slot for displaying text or component related to field.
Baseline implementation has been done for vaadin-text-field. See vaadin/vaadin-text-field#410
All implementations should follow the baseline implementation to achieve the same functionality.

Requirements

• Add helper-text part which contains slot helper
• Add observer to add has-helper attribute when helperText property is set
• Add slotchange event handler to add has-helper attribute when content placed in helper slot
• Add helper text to aria-describedby
• Add observer for error message to add has-error-message attribute (used to hide helper in material theme when error message is set)

Visual specifications