Add placeholder to the scan watcher "filter" GUI

close #801

tomwer/gui/control/datawatcher/datawatcher.py

@@ -266,6 +266,7 @@ class DataWatcherWidget(_DataWatcher, qt.QMainWindow):
         self._filterQLE.setToolTip(
             "You can provide a Linux Regular Expression that will insure only file fitting the expression will be discovered"
         )
+        self._filterQLE.setPlaceholderText("*file_pattern*")
         widget.layout().addWidget(self._filterQLE)
         self._filterQLE.editingFinished.connect(self._restartObservation)
         self._filterQLE.editingFinished.connect(self._filterChanged)