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

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

Some Windows keymaps have bogus mappings for the Ctrl modifier. !4423 attempted
to fix this by ignoring the Ctrl layer, but that 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 + ^). Basically, always ignore Ctrl unless it is used in
conjunction with Alt, i.e. as part of AltGr.

Related issue: #4667