Input type selector on date views filter configuration form should have a label

When you are configuring a date filter you have the option to select a date, or enter a relative date. The form element used to select the input type has no label. The default option is "Select a date", and it is common on many sites to use the first option in a select list as a label for that list. Because of this trend it appears that the select list will allow you to select a date from that list, when in reality you are selecting an input type.

The attached patch adds a label to the input type select box, and changes the first option to "specify a fixed date". I think the label part is necessary, but unsure whether renaming the first option is better or not.