FLUID-5210: Added a merge policy

Adds a merge policy to the composite panel for merging selectorsToIgnore. This is necessary for the case where the composite panel has defined selectorsToIgnore. When the auxBuilder is run it needs to add selectorsToIgnore for the subpanel containers.
Merge branch 'FLUID-5210'

* FLUID-5210:
  FLUID-5210: Added a merge policy