Enhancement: Room Exits GUI - monitor/detect changes in exit data.

Restores "<dummy>" text items to special exits' room Id and command column
entries if any entry is deleted during editing.

Correctly utilises Dialog modified flag (e.g. * in dialog title) and only
enables save button if actual changes have been made. Defines a TExit class (only internally used at
moment) to record details and provide equality and inequality test
operators.

Abstract details of initialising dialog controls {to
void dlgRoomExits::initExit(...) and TExit * makeExitFromControls(...)} to
reduce the amount of code that would have to be explicitly duplicated but
modified for each exit.

Disables sorting of special exit items in table widget otherwise entries
rearrange themselves as the (sorted) data item is edited.

Signed-off-by: Stephen Lyons <slysven@virginmedia.com>