- widens path fields in the spring options tab of the preferences dialog

This changes the BoxSizer flags so that they expands horizontally to fill the size of the preferences dialog, even when it is resized, and provides the corresponding priority weights accordingly.
I think there was some previous confusion about the meaning of the SlPaths::SetUsedSpringIndex and SlPaths::GetCurrentUsedSpringIndex. It seems that these were meant to indicate which engine's unitsync library is currently used by springlobby. They are -not- meant to indicate which engine is currently selected in the list box. The former is model state and the latter is a view state. The user should be able to click through the list and modify the values for each engine without loading the library into the process. The latter functionality belongs to the Apply button.