check for optional gtk3 support
also check for libwnck3, will have to be dependent on gtk3 anyway
set GTK2/GTK3/WNCK/WNCK3 LIBS/CFLAGS depending on USE_GTK3
use gtk_widget_is_visible instead of GTK_WIDGET_VISIBLE()
include gdk/gdkkeysyms-compat.h to get GDK_Menu #define
GTK_WIDGET_SET_FLAGS seems to be gtk2 only
gdk_spawn_command_line_on_screen() was removed, try to use g_app_info_launch()
GtkComboBoxText appeared in gtk 2.24.1, use it if available
GTK_WIDGET_SET_FLAGS seems to be gtk2 only
Use gtk_check_menu_item_get_active() instead of poking directly at the active member
Use gtk_widget_get_window() (appeared in Gtk 2.14) instead of poking at window struct member
Depend on Gtk 2.14, released ages ago...
expose-event was replaced by draw, assign the same callback for now (probably wrong)
[wip] directly pass a hardcoded red GdkColor where gtk2 code pokes at GtkStyle internals
[wip] gets taskmanager to build with Gtk3

- use gtk_widget_get_allocated_width/gtk_widget_get_allocated_height instead of poking at GtkAllocation internals
- use gtk_widget_get_window() instead of poking at window struct member
- correct args for gtk_paint_box/gtk_paint_shadow, probably wrong too but builds for now
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.