ProjectExplorer: Use constants for compile_warning.png and _error

Instead of repeating strings like
   ":/projectexplorer/images/compile_warning.png" all over the code,
we should use constants for it.

Change-Id: I91b36838d6da0a2332381e433788e796948d26db
Reviewed-by: hjk <hjk121@nokiamail.com>
QmlDesigner.PropertyEditor: Remove string helper function

We use C++ helper functions instead.

Change-Id: I03ef168f4e47a6e2437676d906072e5f5ce7377c
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>