TechDraw: Fix dimension alignment issue.

Fixes issue #19121 "dimension numbers placed lower than normal".

Problem:

QGCustomText has a new method `alignmentRect` which can optionally return a
`tightBoundingRect` instead of the regular `boundingRect`. The
`alignmentRect` is used for laying out the `QGIDatumLabel`, however
QGraphicsItemGroup's `childrenBoundingRect` use the childrens'
`boundingRect` and knows nothing of `alignmentRect`. The result is an
improperly sized label and frame and miss alignment. Additionally
`childrenBoundingRect` calculations includes hidden views, so even though
the `m_tolTextOver` and `m_tolTextUnder` are hidden they still affect
the bounding rect size.

Solution:

1. Implement new method `QGIDatumLabel::tightBoundingRect` this
   calculates the bounding rect using the subview's
   `aligmentRect` if there is text in the custom text.
2. Use `tightBoundingRect` in place of `boundingRect` to for the drawing of
   arrows and the exact dim frames.
3. This PR acknowledges that there are some edge case fonts that while not
   clipped may not interface perfectly with arrows and the exact dimension
   frame.
4. Fix vertical alignment of `m_tolTextOver` / `m_tolTextUnder`
5. Incorporate PR Review comments

src/Mod/TechDraw/Gui/QGCustomText.cpp

@@ -178,7 +178,7 @@ void QGCustomText::paint ( QPainter * painter, const QStyleOptionGraphicsItem *
     myOption.state &= ~QStyle::State_Selected;
 
 //    painter->setPen(Qt::green);
-//    painter->drawRect(boundingRect());          //good for debugging
+//    painter->drawRect(alignmentRect());          //good for debugging
 
     QGraphicsTextItem::paint (painter, &myOption, widget);
 }