Background MFE option

[hardl]

Add property to viewGroupSettings entry, so it can be specified that a viewGroup is always present in the background.

If option is true, a viewgroup iframe with its preloadUrl is created on app initialization phase. From an implementation side, it is an enhancement of luigis preloading mechanism (https://github.com/SAP/luigi/blob/master/core/src/services/preloading.js)