Import QucsStudio Filter Synthesis source code
Add GPL informations in modifed files
Removed ActiveFilter. See discussion: https://github.com/Qucs/qucs/pull/15
cleanup qucs-filter/Makefile.am
update qucs-filter/CMakeLists.txt
fix warning deprecation warning

./material_props.h:23:21:
 warning: conversion from string literal to 'pChar' (aka 'char *') is deprecated [-Wc++11-compat-deprecated-writable-strings]
update deprecated Qt header includes