Target setup page: Fix some glitches

- Make sure the "Manage" button is always enabled.
- Clear the list of possible build configs and collapse
  the details widget when a kit becomes unusable.
- Do not collapse the details widget when it gets selected.
- The "Details" button was sometimes grayed out when it shouldn't
  have been.

Change-Id: Ibc0f44213b633aebddfa7542d7dbdd0244df23f9
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/utils/detailswidget.cpp

@@ -229,6 +229,16 @@ void DetailsWidget::setUseCheckBox(bool b)
     d->updateControls();
 }
 
+void DetailsWidget::setCheckable(bool b)
+{
+    d->m_summaryCheckBox->setEnabled(b);
+}
+
+void DetailsWidget::setExpandable(bool b)
+{
+    d->m_detailsButton->setEnabled(b);
+}
+
 void DetailsWidget::setChecked(bool b)
 {
     d->m_summaryCheckBox->setChecked(b);