Add toolTipsVisible feature to KPropertyEditor, if enabled tooltips are visible for property editor items

Tooltips are displayed over each property item, both the name and
value column, and are equal to property descriptions (KProperty::description()).
Tooltips are not displayed for items having empty descriptions.

In addition:

• Do not crash when property set is not assigned and setGroupsVisible or setToolTipsVisible is called
• Rather store view settings (tooltips/groups settings) in view than in model
• Add more checks for presence of the internal model
• Add suitable examples to the KPropertyExample app

FIXED-IN:3.1.0