Make sure to not re-run the query if the filters haven't changed

When changing between the filter line edits in the Browse Data tab, make
sure to only re-run the SQL query if the value of the filter line edits
have actually changed.

See issue #1187.

src/FilterLineEdit.cpp

@@ -30,8 +30,16 @@ void FilterLineEdit::delayedSignalTimerTriggered()
     // Stop the timer first to avoid triggering in intervals
     delaySignalTimer->stop();
 
-    // Emit the delayed signal using the current value
-    emit delayedTextChanged(text());
+    // Only emit text changed signal if the text has actually changed in comparison to the last emitted signal. This is necessary
+    // because this method is also called whenever the line edit loses focus and not only when its text has definitely been changed.
+    if(text() != lastValue)
+    {
+        // Emit the delayed signal using the current value
+        emit delayedTextChanged(text());
+
+        // Remember this value for the next time
+        lastValue = text();
+    }
 }
 
 void FilterLineEdit::keyReleaseEvent(QKeyEvent* event)

src/FilterLineEdit.h

@@ -31,6 +31,7 @@ private:
     QList<FilterLineEdit*>* filterList;
     int columnNumber;
     QTimer* delaySignalTimer;
+    QString lastValue;
 };
 
 #endif