[FIX] Missing Placeholders in MultiComboBox in IE10

- Increasing the z-index of the "simulated" placeholder of the InputBase
solves that problem.
- Fixes also the problem with placeholders inside the MultiInput.

Change-Id: I6412d979de25f0876f667ad063c12c606306b0d4
BCP: 1482007105
Merge "[FIX] Missing Placeholders in MultiComboBox in IE10"