comment out gdk_color_parse() calls, allows taskmanager built with gtk3 to run..
no need for HAVE_GTK3 when you already check for GTK_CHECK_VERSION(2, 24, 1)
use GDK_KEY_Menu instead of the deprecated GDK_Menu via gdkkeysyms-compat.h
Convert GtkBuilder files to gtk3

- replace GtkHBox/VBox by plain GtkBox
- replace GtkHSeparator/VSeparator by plain GtkSeparator
- replace GtkHButtonBox by GtkButtonBox
Save GtkBuilder file with glade 3, allows to display the settings dialog without crashing.
save process-window.ui with glade 3