Searchtools for item list filtering / ordering

### Advantages of the system
* **List models now support a standard JForm for filtering data.** That means that you don't have to render dropdowns manually, check session states for values.....
* **Most managements can almost empty the `populateState()` method.** Now `JModelList` gets the filters and sets the state correctly.
* **All the system is done with layouts**. So if some day my [layouts improvements](https://github.com/joomla/joomla-cms/pull/1419) are merged components can customise the way that they have to show the filters 
* **Now `JHtmlSidebar` supports dynamic layouts**. So it can be used to render anything that contains filters ( a top bar, a custom left bar...)

List of issues, suggestions by @beat:
https://github.com/beat/tracker-jux/issues/1