gdkkeys-win32: Also ignore Ctrl + Shift (etc.)

Isopod requested to merge Isopod/gtk:win32-fix-ctrl-shift into gtk-3-24

It looks like !4423 (merged) was not enough. We also need to ignore combinations of Ctrl with other modifiers, i.e. Ctrl + Shift. For example, Ctrl + Shift + 6 is mapped to the character 0x1E on a US keyboard (but it should be treated as Ctrl + ^, with only Shift being consumed). Basically, always ignore Ctrl unless it is used in conjunction with Alt, i.e. as part of AltGr.

Downstream bug report: Inkscape#3223
Related issue: #4667 (closed).

Merge request reports