Use QScintilla instead of own implementation of a code editor

This is a first rough implementation of QScintilla support as SQL text
editor.

It should work mostly and build fine with qmake as well as cmake. The
new code supports all the features of the old one plus adding a few
subtle improvements. The main point of this, however, is reducing the
code we have to maintain and making it easier to add new features to the
editor.

src/SqlExecutionArea.ui

@@ -14,12 +14,14 @@
    <string>Form</string>
   </property>
   <layout class="QVBoxLayout" name="verticalLayout">
+   <item>
+    <widget class="SqlTextEdit" name="editEditor"/>
+   </item>
    <item>
     <widget class="QSplitter" name="splitter">
      <property name="orientation">
       <enum>Qt::Vertical</enum>
      </property>
-     <widget class="SqlTextEdit" name="editEditor"/>
      <widget class="ExtendedTableWidget" name="tableResult">
       <property name="editTriggers">
        <set>QAbstractItemView::NoEditTriggers</set>
@@ -108,12 +110,12 @@
   </customwidget>
   <customwidget>
    <class>SqlTextEdit</class>
-   <extends>QTextEdit</extends>
+   <extends>QAbstractScrollArea</extends>
    <header>sqltextedit.h</header>
+   <container>1</container>
   </customwidget>
  </customwidgets>
  <tabstops>
-  <tabstop>editEditor</tabstop>
   <tabstop>tableResult</tabstop>
   <tabstop>editErrors</tabstop>
   <tabstop>buttonSave</tabstop>