Improve the inheritance logic as it is currently very limited and does not work intuitively

First of all, 'root' hst:configuration nodes should never 'hide' a 'root' hst:configuration node from a inherited config:

Thus if we have:

demo (hst:configuration) (hst:inheritsfrom = ../common)
hst:pages
a
common (hst:configuration)
hst:pages
a
child_of_a
b

The

1) demo/hst:pages does not 'hide' common/hst:pages
2) demo/hst:pages/a does hide common/hst:pages/a (thus the child child_of_a is not inherited)

The above this results thus in:

demo (hst:configuration) (hst:inheritsfrom = ../common)
hst:pages
a
b

Because we will also have a hstdefault (which has lowest precedence but is inherited implicitly), we will have the following total behaviour:

if our configuration is:

demo (hst:configuration) (hst:inheritsfrom = ../common)
hst:pages
a
common (hst:configuration)
hst:pages
a
child_of_a
b
hstdefault (hst:configuration)
hst:pages
b
child_of_b
c
child_of_c

demo (hst:configuration) (hst:inheritsfrom = ../common)
hst:pages
a (from demo: it does not inherit config below 'a' from common!)
b (from common: it does not inherit config below 'b' from hstdefault!)
c (from hstdefault)
child_of_c