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.

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