FIX(client): Improve handling of the GlobalShortcutButtons dialog

This commits sets the shortcut configuration dialog as a modal window.
It also explicitly uses the MainWindow as a parent, such that the window
is centered over the Mumble application instead of randomly out of bounds
of the screen.

src/mumble/GlobalShortcutButtons.cpp

@@ -10,13 +10,25 @@
 
 #include "Global.h"
 
+#include <QRect>
 #include <QTimer>
 
 GlobalShortcutButtons::GlobalShortcutButtons(QWidget *parent) : QDialog(parent), m_ui(new Ui::GlobalShortcutButtons) {
 	m_ui->setupUi(this);
+	setModal(true);
 
 	connect(m_ui->addButton, &QPushButton::toggled, this, &GlobalShortcutButtons::toggleCapture);
 	connect(m_ui->removeButton, &QPushButton::clicked, this, &GlobalShortcutButtons::removeItem);
+
+	QWidget *dialogParent = parent;
+	while (dialogParent && !qobject_cast< QDialog * >(dialogParent)) {
+		dialogParent = dialogParent->parentWidget();
+	}
+
+	if (dialogParent) {
+		QRect geometry = dialogParent->geometry();
+		move(geometry.center().x(), geometry.center().y());
+	}
 }
 
 GlobalShortcutButtons::~GlobalShortcutButtons() {