Fix frame-focus detection.
Fix frame-focus detection.

In b05276ed8264e5a71f20a7068690ba2a414ee6d8, I inadvertently moved the
focus handler from window to document.  As a consequence, detectFocus
(now registerFocus) wasn't being called when expected.

tuchangwei/vimium,PrestanceDesign/vimium,PickRelated/vimium,tuchangwei/vimium,Aleroniponi/vimium,elegantwww/vimium,cyrixhero/vimium,borgified/vimium,Lw-Cui/vimium,zhangmin510/vimium,dimatter/vimium,mrmr1993/vimium,Robinson10240/vimium,douglas-larocca/vimium,wildeyes/vimium,VPashkov/vimium,smblott-github/vimium,Robinson10240/vimium,bkudria/vimium,decaffeinate-examples/vimium,willsALMANJ/vimium,mrmr1993/vimium,elderbas/vimium,hhsue/vimium,VPashkov/vimium,Lw-Cui/vimium,gdh1995/vimium,decaffeinate-examples/vimium,PrestanceDesign/vimium,poacher2k/vimium,elegantwww/vimium,philc/vimium,wildeyes/vimium,dimatter/vimium,zhangmin510/vimium,elegantwww/vimium,cyrixhero/vimium,wildeyes/vimium,douglas-larocca/vimium,bkudria/vimium,PickRelated/vimium,borgified/vimium,willsALMANJ/vimium,PickRelated/vimium,bkudria/vimium,smblott-github/vimium,Lw-Cui/vimium,dimatter/vimium,cyrixhero/vimium,gdh1995/vimium,Aleroniponi/vimium,mrmr1993/vimium,philc/vimium,justalittlenoob/vimium,tuchangwei/vimium,csmalin/vimium,poacher2k/vimium,douglas-larocca/vimium,philc/vimium,zhangmin510/vimium,csmalin/vimium,VPashkov/vimium,hhsue/vimium,poacher2k/vimium,smblott-github/vimium,borgified/vimium,PrestanceDesign/vimium,justalittlenoob/vimium,Robinson10240/vimium,justalittlenoob/vimium,Aleroniponi/vimium,elderbas/vimium,gdh1995/vimium,csmalin/vimium,elderbas/vimium,decaffeinate-examples/vimium,hhsue/vimium,willsALMANJ/vimium