Added config option richTextMode to TeaserOverlayPropertyFieldBase

When the new option richTextMode is set to false (the default) in class com.coremedia.cms.editor.sdk.premular.fields.teaser.TeaserOverlayPropertyFieldBase the class works as before, that is no richtext editing is possible when no style is selected in the style chooser. Otherwise richtext editing is always possible and the toolbar buttons are enabled.

(CMS-14243)