Editor: reduce complexity of TextDocumentManipulator

Move implementations either to the TextEditorWidget, TextDocument, or to
the caller side. This makes the TextDocumentManipulator basically a
repeater so we can easily replace it with a TextEditorWidget in the
next step.

Change-Id: I04de3cb295b56fbaae99145f8e579fe405b065a5
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/texteditor/textdocument.cpp

@@ -340,6 +340,11 @@ QChar TextDocument::characterAt(int pos) const
     return document()->characterAt(pos);
 }
 
+QString TextDocument::blockText(int blockNumber) const
+{
+    return document()->findBlockByNumber(blockNumber).text();
+}
+
 void TextDocument::setTypingSettings(const TypingSettings &typingSettings)
 {
     d->m_typingSettings = typingSettings;