reworked root path handling / fix of not working stateful forms

[kbachl]

• fix: stateful forms on root path in index.brix wouldnt work as alwa…ys a new page instance was created and the existing one instance was never reused
• simplified root path handling by only 1 if check now (changes above made ajax work, too)
• index.brix may now be referenced in the folder as reference without throwing error like before, so a index.brix page may be referenced by root folder or not, both versions now work (simplification)