Set a default size on first launch

When `~/.config/nvim-qt/window-geometry.conf` does not exist then
restoreGeometry() will initialize a window that is tiny and must be
resized in order to be usable in some environments.

Resize the window to half the screen when launched for the first time.
Do not restore window state when it is invalid or missing.

src/gui/mainwindow.cpp

@@ -1,8 +1,10 @@
 #include "mainwindow.h"
 
+#include <QApplication>
 #include <QCloseEvent>
 #include <QEventLoop>
 #include <QLayout>
+#include <QScreen>
 #include <QSettings>
 #include <QStyleFactory>
 #include <QToolBar>
@@ -14,6 +16,23 @@ static QString DefaultWindowTitle() noexcept
 	return "Neovim";
 }
 
+static void SetDefaultWindowSize(QWidget& widget) noexcept
+{
+	const QPoint local_position{ widget.width() / 2, 0 };
+	const QPoint global_position{ widget.mapToGlobal(local_position) };
+	const QScreen* screen{ qApp->screenAt(global_position) };
+	if (!screen) {
+		return;
+	}
+	const QRect geometry{ screen->availableGeometry() };
+	if (screen->orientation() == Qt::LandscapeOrientation) {
+		widget.resize(geometry.width() / 2, geometry.height());
+	}
+	else {
+		widget.resize(geometry.width(), geometry.height() / 2);
+	}
+}
+
 MainWindow::MainWindow(NeovimConnector* c, QWidget* parent) noexcept
 	: QMainWindow{ parent }
 	, m_tabline{ *c, this }
@@ -333,8 +352,17 @@ void MainWindow::restoreWindowGeometry()
 	if (!settings.value("restore_window_geometry", true).toBool()) {
 		return;
 	}
-	restoreGeometry(settings.value("window_geometry").toByteArray());
-	restoreState(settings.value("window_state").toByteArray());
+	const QVariant geometry{ settings.value("window_geometry") };
+	if (geometry.isValid()) {
+		restoreGeometry(geometry.toByteArray());
+	}
+	else {
+		SetDefaultWindowSize(*this);
+	}
+	const QVariant state{ settings.value("window_state") };
+	if (state.isValid()) {
+		restoreState(state.toByteArray());
+	}
 }
 
 void MainWindow::setGuiAdaptiveColorEnabled(bool isEnabled)