Debug|libdeng2: Added assertions regarding valid use of Observers and Widgets
Fixed|UI|Client: Crash when deleting a popup menu widget

There was an inadvertent duplicate call to the base class’s
preparePanelForOpening() method in PopupMenuWidget.
Fixed|UI|Client: Don’t offer events to the fake mouse cursor widget
UI|ProfilePickerWidget: Conform to common widget behavior of popup menus

Close picker’s menu if it’s already open.