Gropuing file obtained from instrument window always saved to mantid install directory

[abuts]

This minor bug highly decrease Mantid usability

1. Load a workspace (e.g. wish) and show the instrument view (e.g. Y-projection)

2. In instrument view go to pick->pixel selection

3)Select an area of interest by clicking on a place and dragging the mouse pointer selecting an area. Menu appears after releasing the mouse button. Choose Create Grouping file->include (or exclude) does not metter

4. File selection menu appears prompting to pick/name a grouping file

This file selection menu always pointing to Mantid installation area (bin) (at least at windows). If one chooses other area to save the grouping file to, Mantid does not remember the choice and next time one tries to save another grouping file, suggest the Mantid installation folder again.

Necessary:

1. First prompt should point to chosen Mantid save directory
2. Subsequent prompts should point to last selected save location.
3. The saved file should have xml extension as this is xml file if no other extension is explicitly given (currently it writes files without extension)

[abuts]

This issue was originally trac ticket 6128