QQuickShaderEffect's attribute/uniform parser does not support core profile

qt_search_for_variable() in qquickshadereffect.cpp contains code to find attribute and uniform names in a GLSL shader. It is not yet prepared to handle version >=130 that do not use the "attribute" keyword anymore.

5.3 & 5.4 are supposed to bring the last missing pieces of core profile support in Quick 2 & Qt Quick Controls and therefore this needs to be corrected too.