ProjectExplorer: Fixing style of read-only SysRootInformationConfigWidget

Style of read-only SysRootInformationConfigWidget and QmakeKitConfigWidget
was not matching.

SysRootInformationConfigWidget was using lineEdit.setReadOnly(true)
QmakeKitConfigWidget and even other *InformationConfigWidgets are
using lineEdit.setEnabled(false)

Therefore making SysRootInformationConfigWidget to use setEnabled(false)
too.

Change-Id: I637b7a7afe522b7a622975a1b37b2256a802ae1a
Reviewed-by: Mehdi Fekari <mfekari@blackberry.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/projectexplorer/kitinformationconfigwidget.cpp

@@ -89,7 +89,7 @@ void SysRootInformationConfigWidget::refresh()
 
 void SysRootInformationConfigWidget::makeReadOnly()
 {
-    m_chooser->setReadOnly(true);
+    m_chooser->setEnabled(false);
 }
 
 QWidget *SysRootInformationConfigWidget::mainWidget() const