MainWindow: Add Ctrl+Return shortcut for executing SQL and add tooltips

Add the Ctrl+Return shortcut for executing the current SQL again. This
needs to be done using C++ code instead of setting it in Qt Designer
because setting multiple shortcuts in the latter means defining a key
sequence row.

Also add the keyboard shortcuts to the tooltips of the buttons as they
didn't appear anywhere in the program.

src/MainWindow.cpp

@@ -79,6 +79,11 @@ void MainWindow::init()
     ui->dbTreeWidget->setColumnHidden(1, true);
     ui->dbTreeWidget->setColumnWidth(0, 300);
 
+    // Add keyboard shortcuts
+    QList<QKeySequence> shortcuts = ui->actionExecuteSql->shortcuts();
+    shortcuts.push_back(QKeySequence(tr("Ctrl+Return")));
+    ui->actionExecuteSql->setShortcuts(shortcuts);
+
     // Create the actions for the recently opened dbs list
     for(int i = 0; i < MaxRecentFiles; ++i) {
         recentFileActs[i] = new QAction(this);