Replace XPath queries

[bjarnef]

Depending on the content structure we can look for a settings node at root, under home node or deeper in structure.
Alternatively we could also use Descendant() method, DescendantOfType() or FirstChildOfType().

Fixes #9

[idseefeld]

@bjarnef Hi Bjarne, I have just tried this patch and found two issues:

1. I had to add using Umbraco.Extensions; to fix home?.FirstChild(... in line 18
2. The tests for SiteSettings do not pass. Probably because MockContentQueryXPAth is used 🤔

[bjarnef]

@idseefeld yes, you're are right. The namespace Umbraco.Extensions was missing. Fixed in d439fae

Not sure about the test for SiteSettings though.

[Adolfi]

Merged and fixed failing tests. Thanks @bjarnef !!