Combobox enhancements to be used as auto-complete widget

[wkeese]

Enhancements to Combobox so it can be used as an auto-complete widget.

• The widget is designed to be friendly for server-side stores.
• The widget now accepts a source. This will be assigned to the list when required.
• The widget exposes a defaultQuery property. This can be a function or an object and consists in the default query (!== {}).
• The widget can have an initial value, accessible through widget.value. It can be an array or a string. See `samples/Combobox.html for few examples.
• The widget can behave as an auto-complete widget (set through hasDownArrow=false). In this scenario, the user can type some text and matching items will be shown. The auto-complete version works in mobile too, showing a popup with the a title and the input element. This mode works only when autoFilter=true and selectionMode=single.
• Add minFilterChars property. It defines the minimum number of characters the user has to type before a filtering operation can run.
• Add filterDelay property. It defines the amount of time (from the last key pressed) has to pass before a filtering operation gets executed.
• Bugfixes.