a11y: Orca gets control character strings in key events
When typing e.g. control-tab, the string part of the key event sent to at-spi by gtk2/3 contains "\t" instead of "Tab", while pressing tab alone brings "Tab". The "\t" behavior is not useful to a screen reader (see https://mail.gnome.org/archives/orca-list/2019-February/msg00315.html), gtk should just consistently pass "Tab". I will submit a merge request for this.