Fix getting stuck in normal mode in docked DevTools after refocusing

Fix getting stuck in normal mode in docked DevTools after refocusing

In Nightly 70 the docked DevTools were switched from a Chrome Frame to a
Content Frame. The part of VimFx that determines what is part of the
browser UI (to work around a bug regarding delayed ipc messages) then
didn't recognize the DevTools as part of the UI any longer.

When a focusType event comes from the browser UI, a second event is
fired erroneously a few milliseconds later. This caused us to enter
insert mode in the devtools, and immediately exit out of it again,
making it impossible to type in e.g. the style editor.

STR:
    1. open devtools->style editor (or ->network)
    2. focus textbox (vimfx icon will turn silver)
    3. focus webpage (vimfx switches to normal mode)
    4. focus style editor (vimfx fails to return to ignore mode)

regressed by: Bug 1539979 - https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=91f7769b3d7ec17aa11f7a8ada7104f5b4cec38d&tochange=39e19afc797d310f568447eae7054789e641b591

Unrelated to this change, the about:devtools-toolbox's chrome document
was renamed from .xul to .xhtml a few releases back. This second URI was
added; no idea if its absence broke anything.