[xbase][contentAssist] Fixed missing proposals when getters and fields are present

see https://bugs.eclipse.org/bugs/show_bug.cgi?id=448017

Change-Id: I41955c965fbb059258a167ed9a0ca05c65c11d3d
Test for https://bugs.eclipse.org/bugs/show_bug.cgi?id=428928
Merge "Test for https://bugs.eclipse.org/bugs/show_bug.cgi?id=428928"