[FEATURE] Add hasSubpages condition ViewHelper

Using this ViewHelper it is possible to add some extra markup to wrap a sub-navigation in without having to use the menu ViewHelper which can be a little overhead.
[TASK] Adapt refactoring of condition ViewHelpers
[TASK]  Guard against empty/zero page UID passed to pageSelect
Merge pull request #168 from bjo3rnf/subpages

[FEATURE] Add hasSubpages condition ViewHelper