GdkWin32Keymap bugfixes

I accumulated a couple of bugfixes over the last days and combined them in this MR.

• Fix certain keyboard shortcuts not working when capslock is active (See Inkscape #3082)
• Prevent crash after loading of keyboard DLL failed (See #4610 (comment 1350259))
• Add a keysym mapping for the capslock key
• Fix handling of SGCAPS (Swiss German Caps)
• Perform keyboard layout substitution where necessary (see #4610 (comment 1350803))

You can read more details in the commit messages.