patch 9.1.0058: Cannot map Super Keys in GTK UI

patch 9.1.0058: Cannot map Super Keys in GTK UI

Problem:  Cannot map Super Keys in GTK UI
          (Casey Tucker)
Solution: Enable Super Key mappings in GTK using <D-Key>
          (Casey Tucker)

As a developer who works in both Mac and Linux using the same keyboard,
it can be frustrating having to remember different key combinations or
having to rely on system utilities to remap keys.

This change allows `<D-z>` `<D-x>` `<D-c>` `<D-v>` etc. to be recognized
by the `map` commands, along with the `<D-S-...>` shifted variants.

```vimrc
if has('gui_gtk')
	nnoremap  <D-z>    u
	nnoremap  <D-S-Z>  <C-r>
	vnoremap  <D-x>    "+d
	vnoremap  <D-c>    "+y
	cnoremap  <D-v>    <C-R>+
	inoremap  <D-v>    <C-o>"+gP
	nnoremap  <D-v>    "+P
	vnoremap  <D-v>    "-d"+P
	nnoremap  <D-s>    :w<CR>
	inoremap  <D-s>    <C-o>:w<CR>
	nnoremap  <D-w>    :q<CR>
	nnoremap  <D-q>    :qa<CR>
	nnoremap  <D-t>    :tabe<CR>
	nnoremap  <D-S-T>  :vs#<CR><C-w>T
	nnoremap  <D-a>    ggVG
	vnoremap  <D-a>    <ESC>ggVG
	inoremap  <D-a>    <ESC>ggVG
	nnoremap  <D-f>    /
	nnoremap  <D-g>    n
	nnoremap  <D-S-G>  N
	vnoremap  <D-x>    "+x
endif
```

closes: #12698

Signed-off-by: Casey Tucker <dctucker@hotmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>