Improve event filtering GUI.  Refs #6556.

Improvements include:
1. Add event handling to disable/enable widgets if different filtering
approach is selected;
2. Clean the GUI.
3. Add filter info as a helping message.
Improve powder data reduction GUI.  Refs #6555.

Improvements include:
1. Move preserve-events and file extension to advanced setup tab;
2. Clean GUI;
3. Add helping infomation;
4. Add tool tip to each widget.