Introduced horizontal and vertical spacing in grid layout.
Child items of DeclarativeItem are properly destroyed.

This happens recursively over the DeclarativeItem's elements/forms.
Builder pattern allows chaining of methods used at initialization.
addElement is a much more descriptive name than add (GridLayoutElement)
Add horizontal and vertical alignment specification to GridLayoutElement
Add row and column span, as well as stretch factors to the grid layout

The new properties are not used yet.
Syntax of addElement of GridLayoutElement is consistent with syntax for
calling addItem of QGridLayout.
Private variables of GridLayoutElement have better names.
It is now possible to modify the stretch factors.
Use stretch factors, no merged cells, use additionally available space
Handle merged cells properly.
Test grid layout with cell merging and stretch factors.