Store last clipboard register type

This makes `yy` -> `p` line copying work correctly, but whether external
text would be pasted as whole lines or not now depends on the last copy
operation.

The proper implementation would store the register type in the actual
Gtk clipboard, but gtk-rs is not ready for that yet.

runtime/plugin/nvim_gui_shim.vim

@@ -5,10 +5,12 @@ endif
 let g:GuiLoaded = 1
 
 if exists('g:GuiInternalClipboard')
+	let s:LastRegType = 'v'
 	function! provider#clipboard#Call(method, args) abort
 		if a:method == 'get'
-			return [rpcrequest(1, 'Gui', 'Clipboard', 'Get', a:args[0]), 'v']
+			return [rpcrequest(1, 'Gui', 'Clipboard', 'Get', a:args[0]), s:LastRegType]
 		elseif a:method == 'set'
+			let s:LastRegType = a:args[1]
 			call rpcnotify(1, 'Gui', 'Clipboard', 'Set', a:args[2], join(a:args[0], '
'))
 		endif
 	endfunction