Permit creation of ports using toolbar for component diagram:

umbrello/widgets/toolbarstateonewidget.cpp
- In function mouseReleaseWidget() admit
currentWidget()->baseType() == WidgetBase::wt_Component
to the call of setWidget(currentWidget())
- In function setWidget() handling of widgetType() == WidgetBase::wt_Pin,
add handling of m_firstObject->baseType() == WidgetBase::wt_Component:
- Open a dialog querying the user for a new port name
- If user confirmed port creation then
- Let Object_Factory create a UML Port object of given name.
The object is owned by the UML Component underlying m_firstObject.
- Let Widget_Factory create a PortWidget representing the UML Port and
assign this widget to `umlwidget'.

umbrello/worktoolbar.cpp
- In function slotCheckToolBar(Uml::DiagramType::Enum) switch (m_Type) case
Uml::DiagramType::Component, insertHotBtn(tbb_Pin).
tbb_Pin is reused for ports.

umbrello/widgets/portwidget.cpp
- In function attachToOwningComponent() prevent the port from being dropped
underneath its owning component (thus becoming inaccessible).
This is achieved as follows:
If scenePos.x() is in the range
[ owner->x() : owner->x() + owner->width() ]
and scenePos.y() is in the range
[ owner->y() : owner->y() + owner->height() ]
then
if scenePos.y() is in the top half of owner then
setY(owner->y() - height())
else
setY(owner->y() + owner->height())