Use QSize/QPoint to handle and store coordinate values (size and position)

Previously, the rules model stored the "coordinate" type properties as a
QString with format x, y.

This fails when setting the properties to the config schema, as it requires
a proper QPoint or QSize value, specially the latter which can't be
convert from such a string.

BUG: 421055
FIXED-IN: 5.19.0

• Add a new rule and set its position and size properties
• Hitting apply stores the right values in ~\.config\kwinrulesrc
• Close the kcm and reopen, the values are loaded
• Property detection still works for size and position

Please note that there is a pre-existing bug of some position/sizes not being
applied to the windows in some cases, when using Apply Initially.
Better try using the Force policy.