Don't use ASCII control characters to denote the input of Esc, Tab,
Return/Enter, Backspace and Delete, as it seems that it is not how
Windows handle them, and they cause weird characters to appear in the
input field on GTK+3 programs in non-English Windows.  Instead, let
these keys be handled as-is on Windows, like what is done in GTK+-2.x.

Checked with mclasen on IRC, and thanks to the people who verified the
patch to not break anything on English Windows.