fix: Use selection background instead of underline for occurrences

This fixes Megaxela#11.

This is only a workaround. The QFramedTextAttribute solution seems not
possible, because we need to insert QChar::ObjectReplacementCharacter
for every occurrence, thus ruin the undo history.

It will be better if the user can set a custom color for this.

Author: ouuan

src/internal/QCodeEditor.cpp

@@ -448,7 +448,7 @@ void QCodeEditor::highlightOccurrences()
                 {
                     QTextEdit::ExtraSelection e;
                     e.cursor = cursor;
-                    e.format.setFontUnderline(true);
+                    e.format.setBackground(m_syntaxStyle->getFormat("Selection").background());
                     extra2.push_back(e);
                 }
                 cursor = doc->find(text, cursor, QTextDocument::FindWholeWords | QTextDocument::FindCaseSensitively);