COMP: Support building DICOMWidget against Qt 5.12.8

Remove setting of unsupported and unused `placeholderText` property from
`ctkDICOMVisualBrowserWidget.ui`. Since the property is set to its default
value, removing it will not impact how the widget is rendered. For
reference, the use of `placeholderText` was originally introduced in
88ff72b ("ENH: Add Visual DICOM Browser (commontk#1165)", 2024-01-19)

Conditionally use `Qt::MatchRegularExpression` introduced in Qt 5.15.
For reference, its use was originally introduced in 888cdd9 ("ENH: Add
ctkDICOMJobListWidget for logging jobs activity in the UI", 2024-01-18)

Libs/DICOM/Widgets/Resources/UI/ctkDICOMVisualBrowserWidget.ui

@@ -403,9 +403,6 @@
                 <property name="sizeAdjustPolicy">
                  <enum>QComboBox::AdjustToContents</enum>
                 </property>
-                <property name="placeholderText">
-                 <string/>
-                </property>
                 <property name="frame">
                  <bool>true</bool>
                 </property>

Libs/DICOM/Widgets/ctkDICOMJobListWidget.cpp

@@ -402,7 +402,12 @@ void QCenteredItemModel::setProgressBar(int row, const ctkDICOMJobDetail &td, ct
 //----------------------------------------------------------------------------
 void QCenteredItemModel::clearCompletedJobs()
 {
+#if (QT_VERSION >= QT_VERSION_CHECK(5,15,0))
   QList<QStandardItem*> list = this->findItems(tr("completed"), Qt::MatchRegularExpression, Columns::Status);
+#else
+  QList<QStandardItem*> list = this->findItems(tr("completed"), Qt::MatchRegExp, Columns::Status);
+#endif
+
   foreach (QStandardItem* item, list)
   {
     this->removeRow(item->row());