[FEATURE] Option in TS constant to prevent "parent decides" field display

By setting plugin.tx_fluidpages.siteRootInheritance = 0 (default value is 1, a value not set is considered 1 as well) the "parent decides" field for "page layout - this page" is hidden if the page being edited is a site root (is_siteroot = 1).
[TASK] Hide "parent decides" option for pages in pid zero.
Merge pull request #50 from NamelessCoder/selectorinherit

[FEATURE] Options and behavior to hide "parent decides" for root-type pages