Make IE engine Hi-DPI aware.

... so that zooming levels are the same for both Firefox (>=22) and
IE(>=8) engines.
See msdn.microsoft.com/en-us/library/cc849094(v=vs.85).aspx#OptInHighDPI
for more information.
Enable DPI-aware zooming.
Only auto-switch engine for root windows.

... so that we won't switch on "Get Add-ons" page in the Add-ons
Manager.
Suppress findbar warnings on Nightly 32
Make sure to zoom in high-DPI environment.
Fix: forward button not visible on Firefox 29+