Avoid adding <option select-all> to original <select>, minor improvements

- don't change original $select any more (adding -select all- option): this alters POST-data and may break existing external JS 
- Button says -all- when all elements are selected 
- Option to add -select all- automatically if more than x options are present 
- buttonText() only returns text itself, html for caret is added in updateButtonText()
- extended label width: complete line (even space next to label-text) will accept clicks