Add a text field to filter CMake variables in the CMake project
configuration view. As in the cmake-gui, the filter does a simple
string search over all variable names and values.

Task-number: QTCREATORBUG-17973
Change-Id: Id7219c16509c620c39978cb05c5e0e62cf9c19df
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>