Fixed 3D plot dialog fields being superscripted.

Using <sub>text</sub> would cause text to be subscript next time the
title or axis properties tabs were opened, which cause the fields to
disappear from the plot. This commit fixes that issue.

Refs #8256

Code/Mantid/MantidPlot/src/Plot3DDialog.cpp

@@ -159,6 +159,7 @@ void Plot3DDialog::initAxesPage()
     QGridLayout *gl1 = new QGridLayout();
     gl1->addWidget(new QLabel(tr("Title")), 0, 0);
 	boxLabel = new QTextEdit();
+	boxLabel->setTextFormat(Qt::PlainText);
 	boxLabel->setMaximumHeight(60);
     gl1->addWidget(boxLabel, 0, 1);
     gl1->addWidget(new QLabel(tr("Axis Font")), 1, 0);
@@ -212,6 +213,7 @@ void Plot3DDialog::initTitlePage()
 
 	QVBoxLayout* vl = new QVBoxLayout();
 	boxTitle = new QTextEdit();
+	boxTitle->setTextFormat(Qt::PlainText);
 	boxTitle->setMaximumHeight(80);
 	vl->addWidget(boxTitle);