Only check for wnck3 if --enable-gtk3 was passed, and enable it by default then.

Only check for wnck/gksu in the other case.
This way one only needs --enable-gtk3 to switch.
use gtk_tool_button_set_icon_name() instead of gtk_tool_button_set_stock_id()
use a plain GtkBox for the statusbar
use gtk_tool_button_set_icon_name() instead of gtk_tool_button_set_stock_id()
icon-name property was deprecated in gtk3
Rename gtkbuilder files with a -gtk3 suffix for clarity
Restore gtk2 versions of the gtkbuilder ui files
Add automake plumbing to generate both gtk2 and gtk3 ui files in maintainer mode.

- add the resulting files to BUILT_SOURCES & xfce4_taskmanager_SOURCES
- add the gtk3 ui files to EXTRA_DIST
use #ifdef instead of #if to avoid warnings with gtk2
include the gtk3 generated ui file when building against gtk3. This way both flavors build