Search completion; implement <Enter> timer.
Search completion; implement <Enter> timer.

With suggested completions appearing in the vomnibar, the meaning of
<Enter> can change when the completion list is updated, which happens
asynchronously.  The user can end up selecting something they didn't
intend to ... which is bad.

This implements a temporary fix.  When the completion list updates (and
the update might affect what the user expects <Enter> to do), we block
<Enter> for a short time (250ms).  That's pretty bad too.

These UX issues around search completion are proving difficult to get
right.  I will open an issue to discuss them soon.

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