Fix find-mode hangs.
Fix find-mode hangs.

"/" followed immediately by "i" can hang Vimium.

The problem is that launching find mode is asynchronous (we wait until
the HUD is available). Because normal mode is still active, we can enter
insert mode *before* the find-mode HUD receives the focus.

The result is that we end up in both find mode and insert mode, the HUD
has the focus, but the HUD is in insert mode, so it ignores keyboard
events (including `Escape`).

The only way out is to click the page's body and then type `Escape`.

This commit demonstrates the problem: 7d2b00411eae3293fa4c7b1f61b384c0c495b5a2.

This happens in practice, for example while a busy page is loading.

This commit fixes this by ensuring that find-mode blocks keyboard events
immediately (and synchronously) on launch.

philc/vimium,willsALMANJ/vimium,gdh1995/vimium,gdh1995/vimium,philc/vimium,decaffeinate-examples/vimium,smblott-github/vimium,poacher2k/vimium,gdh1995/vimium,willsALMANJ/vimium,philc/vimium,poacher2k/vimium,mrmr1993/vimium,mrmr1993/vimium,willsALMANJ/vimium,smblott-github/vimium,decaffeinate-examples/vimium,mrmr1993/vimium,poacher2k/vimium,smblott-github/vimium,decaffeinate-examples/vimium