COMP: Ensure DICOMWidget compatibility with Qt 5.12.8

This commit resolves compilation issues encountered when building the DICOMWidget
against Qt 5.12.8. The changes include:

1. Removed the setting of the unsupported and unused `placeholderText` property
   from `ctkDICOMVisualBrowserWidget.ui`. This property's removal does not affect
   widget rendering. The removal addresses a compilation error introduced in commit
   88ff72b ("ENH: Add Visual DICOM Browser (#1165)", 2024-01-19):

    ```
    /path/to/CTK/Libs/DICOM/Core/ctkDICOMStorageListener.cpp:71:18: error: ‘acceptAssociations’ is not a member of ‘DcmSCP’
       71 |   return DcmSCP::acceptAssociations();
          |                  ^~~~~~~~~~~~~~~~~~
    ```

2. Introduced conditional utilization of `Qt::MatchRegularExpression`, available
   in Qt 5.15, to resolve a compilation error introduced in
   commit 888cdd9 ("ENH: Add ctkDICOMJobListWidget for logging jobs activity in the UI", 2024-01-18):

    ```
    /path/to/CTK/Libs/DICOM/Widgets/ctkDICOMJobListWidget.cpp:405:69: error: ‘MatchRegularExpression’ is not a member of ‘Qt’
    405 |   QList<QStandardItem*> list = this->findItems(tr("completed"), Qt::MatchRegularExpression, Columns::Status);
        |                                                                     ^~~~~~~~~~~~~~~~~~~~~~
    ```

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());