IMContextSimple: Fix AltGr not working on Win32 [GTK3]

Isopod requested to merge Isopod/gtk:win32-fix-altgr-gtk3 into gtk-3-24

This probably didn't affect many people, as IMContextIME is used by default, but AltGr did not work with IMContextSimple because the code assumed that any key press containing Ctrl or Alt cannot be regular text input. This is not correct on Win32 as AltGr = Ctrl + Alt.

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