Date & time field support for Formbuilder

### Description
Added a datetime button to the Formbuilder module, to create a date or time input field inside your form (for simple reservation forms, etc).

This adds a new icon and button to the Formbuilder module with "date & time" support. You can enter the label, a default value, required checkbox and most importantly: a dropdown where you can choose between a date or time field. You'll get 

### Discussion
I find this a useful update to the Formbuilder **but it needs discussion because of a few problems**:

* ~~I use the Spoon `addDate` and `addTime` methods to create the input element. However, these methods don't seem to support the new HTML5 types: `type="date"` and `type="time"`. Spoon actually creates plain textfields, so no HTML5 date & time fields: the html5 datepicker doesn't appear, the time field doesn't have an up/down button. So currently you don't have HTML5 fields in this PR but you can add a date or time field to your form, and it adds the necessary class names for the jQuery UI datepicker. But ideally I'd love to see a HTML5 datepicker by default, and add the jQuery UI as a fallback datepicker.~~ I started using `addText` because then I can use `setAttribute('type', 'date')` to switch to a html5 date or time field. The type attribute is forbidden with `addDate` so that's why I need to use `addText`. So now you have html5 date & time fields in the frontend. I added some js code to check if the date type is supported, if not add a jquery datepicker fallback. The HTML5 date type automatically uses the most appropriate date format (dd/mm/yy) for your browser & location, but the jQuery datepicker needs a date format. It's currently hardcoded to dd/mm/yy but I should probably add it as a setting in the dialog? Same for "firstday" of the week.
* What should I do with the validation dropdown? Keep it or remove it? The HTML5 or jquery fallback datepicker automatically keeps the input validated right? But with no js or html5 field, it should validate the date or time somehow?
* The default value field isn't very useful except for time. However it could have more functionality, e.g. some way to use "+1 weeks" or something as default value. (If you don't fill in the default value, the Spoon addDate or addTime method uses the current date or time.)

### Screenshots
Some screenshots in dutch:
![image](https://cloud.githubusercontent.com/assets/1352979/4518364/d7fbe7b8-4c86-11e4-938c-eb33204c261d.png)

![image](https://cloud.githubusercontent.com/assets/1352979/4518380/46ffab3a-4c88-11e4-8e5e-35b492efe04c.png)

HTML5 datepicker (Chrome):
![image](https://cloud.githubusercontent.com/assets/1352979/4518685/47f157b2-4c99-11e4-86ba-19b0064a2e88.png)

jQuery fallback (tested in Safari & IE):
![image](https://cloud.githubusercontent.com/assets/1352979/4518389/3be87e6a-4c89-11e4-83b8-d6dc2883e441.png)

This line (and some others) are way too long (check PSR standards: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)