Remove close buttons on tabs and add button for closing SQL tabs

This gives us some more space in the tab bar and should reduce the risk
of accidentally closing a tab. It looks prettier too. We could add an
option for this in the Preferences dialog but in order to not add too
many rarely used options, let's try it without an option first.

See issue #1968.

src/MainWindow.cpp

@@ -333,6 +333,16 @@ void MainWindow::init()
     QShortcut* setTab4Shortcut = new QShortcut(QKeySequence("Alt+4"), this);
     connect(setTab4Shortcut, &QShortcut::activated, [this]() { ui->mainTab->setCurrentIndex(3); });
 
+    // Add button to close Execute SQL tabs
+    QToolButton* buttonCloseSqlTab = new QToolButton(ui->tabSqlAreas);
+    buttonCloseSqlTab->setIcon(QIcon(":icons/close"));
+    buttonCloseSqlTab->setToolTip(tr("Close current SQL tab"));
+    buttonCloseSqlTab->setAutoRaise(true);
+    connect(buttonCloseSqlTab, &QToolButton::clicked, [this]() {
+        closeSqlTab(ui->tabSqlAreas->currentIndex());
+    });
+    ui->tabSqlAreas->setCornerWidget(buttonCloseSqlTab);
+
     // If we're not compiling in SQLCipher, hide its FAQ link in the help menu
 #ifndef ENABLE_SQLCIPHER
     ui->actionSqlCipherFaq->setVisible(false);

src/MainWindow.ui

@@ -39,9 +39,6 @@
       <property name="currentIndex">
        <number>0</number>
       </property>
-      <property name="tabsClosable">
-       <bool>true</bool>
-      </property>
       <property name="movable">
        <bool>true</bool>
       </property>
@@ -727,9 +724,6 @@ You can drag SQL statements from an object row and drop them into other applicat
           <property name="currentIndex">
            <number>-1</number>
           </property>
-          <property name="tabsClosable">
-           <bool>true</bool>
-          </property>
           <property name="movable">
            <bool>true</bool>
           </property>