Code cleanup, improve default WM theme.

mcs-plugin/xfwm4_plugin.h

po/xfwm4.pot

src/events.c

themes/default/close-active.xpm

themes/default/close-inactive.xpm

themes/default/close-pressed.xpm

themes/default/hide-active.xpm

themes/default/hide-inactive.xpm

themes/default/hide-pressed.xpm

themes/default/maximize-active.xpm

themes/default/maximize-inactive.xpm

themes/default/maximize-pressed.xpm

themes/default/maximize-toggled-active.xpm

themes/default/maximize-toggled-inactive.xpm

themes/default/maximize-toggled-pressed.xpm

themes/default/menu-active.xpm

themes/default/menu-inactive.xpm

themes/default/menu-pressed.xpm

themes/default/shade-active.xpm

themes/default/shade-inactive.xpm