JBIDE-18543 If user toggles between Browser Engine options in Browsersim preferences - duplicate error messages are displayed

changed a way of showing error message
JBIDE-17404 CordovaSim cannot be launched on RHEL7

setting GTK3 as default for RHEL7
JBIDE-18519 Cannot run Browsersim on RHEL6 with GTK3 - error instructs user to install package that is already installed

error message for GTK3 changed