Improve the mantid workbench main settings window

[jhaigh0]

Currently, any changes you make in the settings widget immediately write to file with no way to undo.

The settings widget should be changed to have 'Apply', 'Ok', and 'Cancel' buttons like you'd typically find in an interface like this.

This might be best solved by switching to an MVP implementation where the model can hold the potential new state for the settings.