Refocus focused window for FOCUS_IN events on the root window. (#3097)

This deals with (admittedly somewhat misbehaving) clients which
use XSetInputFocus to take focus, but then don't properly restore
focus. This has been observed with TK apps, but also, e.g., Steam.

fixes #2722
fixes #3096

include/xcb.h

@@ -53,6 +53,7 @@
                                                                   ConfigureNotify */                  \
                          XCB_EVENT_MASK_POINTER_MOTION |                                              \
                          XCB_EVENT_MASK_PROPERTY_CHANGE |                                             \
+                         XCB_EVENT_MASK_FOCUS_CHANGE |                                                \
                          XCB_EVENT_MASK_ENTER_WINDOW)
 
 #define xmacro(atom) xcb_atom_t A_##atom;