Feature - Ungrouped Area

[andreicorpo]

Hello, guys.

I opened this PR in order to add a feature that I felt was missing and that is the ability to view ungrouped items in a different area.

Here is a small example of how it looks:

The following options were added:

• showUngroupedItems which displays the mentioned area.
• ungroupedHeaderTemplate which allows the user to customise the look on the header of the ungrouped area.

The area can be hidden/shown by pressing the header of the area.

Looking forward to the discussions that will start here. 😄

Thanks and have a nice day!