Add GuiWindowOpacity command

Directly maps to QWidget::setWindowOpacity().
Takes a double as an argument, the valid range is 0.0 to 1.0.

src/gui/mainwindow.cpp

@@ -103,6 +103,8 @@ void MainWindow::init(NeovimConnector *c)
 			this, &MainWindow::neovimFullScreen);
 	connect(m_shell, &Shell::neovimGuiCloseRequest,
 			this, &MainWindow::neovimGuiCloseRequest);
+	connect(m_shell, &Shell::neovimOpacity,
+			this, &MainWindow::setWindowOpacity);
 	connect(m_nvim, &NeovimConnector::processExited,
 			this, &MainWindow::neovimExited);
 	connect(m_nvim, &NeovimConnector::error,

src/gui/runtime/plugin/nvim_gui_shim.vim

@@ -232,3 +232,8 @@ function! s:GuiScrollBar(enable) abort
 endfunction
 
 command! -nargs=1 GuiScrollBar call s:GuiScrollBar(<args>)
+
+function! s:GuiWindowOpacityCommand(value) abort
+  call rpcnotify(0, 'Gui', 'WindowOpacity', a:value)
+endfunction
+command! -nargs=1 GuiWindowOpacity call s:GuiWindowOpacityCommand("<args>")

src/gui/shell.cpp

@@ -816,6 +816,12 @@ void Shell::handleNeovimNotification(const QByteArray &name, const QVariantList&
 
 			qDebug() << "Neovim changed clipboard" << data << type << reg_name << clipboard;
 			QGuiApplication::clipboard()->setMimeData(clipData, clipboard);
+		} else if (guiEvName == "WindowOpacity" && args.size() == 2) {
+			bool ok = false;
+			auto val = args.at(1).toDouble(&ok);
+			if (ok) {
+				emit neovimOpacity(val);
+			}
 		} else if (guiEvName == "ShowContextMenu") {
 			emit neovimShowContextMenu();
 		}

src/gui/shell.h

@@ -64,6 +64,7 @@ class Shell: public ShellWidget
 	void neovimResized(int rows, int cols);
 	void neovimAttached(bool);
 	void neovimMaximized(bool);
+	void neovimOpacity(double);
 	void neovimSuspend();
 	void neovimFullScreen(bool);
 	void neovimGuiCloseRequest();