Create separated candidate process to preload NUI (#245)

Create separated candidate process to preload NUI (#245)

NUI cannot be preloaded with ElmSharp because those two UI related
module create window while preloading.

When multiple windows are created, it not only increases the memory usage,
but also affects the launching effect and splash screen.

So, create new candidate process for NUI

Assumption
  NUI app has their own apptype "dotnet-nui"
  There are two candidate process. One is for "ElmSharp" and The other is for "NUI"
  XSF is related to "ElmSharp"

Basic Operation
  Set extra value "TIZEN_UIFW" to bundle by loader script. (ElmSharp and NUI)
  Get extra value from bundle in the *__loader_create_cb function and set that to environment.
  Get environment in the preload() method in the Tizen.Runtime.cs
  Skip "ElmSharp" or "NUI" preloading by TIZEN_UIFW