Ghost exits left in the Exits dialog when a room is deleted

Steps to reproduce:

1) delete room A that connects to room B by right-clicking on it and selecting 'delete'
2) right-click on room B and select Exits. Mudlet will crash.

Stacktrace:

0 QString::trimmed qstring.cpp 4470 0x7ffff54080f1
1 dlgRoomExits::initExit dlgRoomExits.cpp 1471 0x4c5ed7
2 dlgRoomExits::init dlgRoomExits.cpp 1538 0x4c694f
3 T2DMap::slot_setExits T2DMap.cpp 3192 0x5b6ca2
4 T2DMap::qt_static_metacall moc_T2DMap.cpp 233 0x6b31d1
5 QMetaObject::activate qobject.cpp 3569 0x7ffff557d2a6
6 QMetaObject::activate qobject.cpp 3444 0x7ffff557d7d7
7 QAction::triggered moc_qaction.cpp 356 0x7ffff6458a52
8 QAction::activate qaction.cpp 1175 0x7ffff645af38
9 QMenuPrivate::activateCausedStack qmenu.cpp 977 0x7ffff65cba7a
10 QMenuPrivate::activateAction qmenu.cpp 1054 0x7ffff65d0b54
11 QMenu::mouseReleaseEvent qmenu.cpp 2359 0x7ffff65d4eaa
12 QWidget::event qwidget.cpp 7946 0x7ffff649aea5
13 QMenu::event qmenu.cpp 2472 0x7ffff65d564b
14 QApplicationPrivate::notify_helper qapplication.cpp 3486 0x7ffff645fc8c
15 QApplication::notify qapplication.cpp 3020 0x7ffff6465b61
16 QCoreApplication::notifyInternal qcoreapplication.cpp 881 0x7ffff5555c2d
17 sendSpontaneousEvent qcoreapplication.h 235 0x7ffff64639e1
18 QApplicationPrivate::sendMouseEvent qapplication.cpp 2507 0x7ffff64639e1
19 QWidgetWindow::handleMouseEvent qwidgetwindow.cpp 404 0x7ffff64b796a
20 QWidgetWindow::event qwidgetwindow.cpp 173 0x7ffff64b99bb
... <More>

Determined that commit 94c5066613e97705a65ec215208609ef35a2f6e8 "Speed up room deletion" uses a new TRoomDB::reverseExitMap to work out which rooms have exits into a room being deleted. This is supposed to short-cut the need to iterating thought every exit for every room to find the exit that leads to a room being deleted so they can be reset to -1 "no exit". Unfortunately NO ATTEMPT IS MADE TO MAINTAIN THE DATA IN TRoomDB::reverseExitMap, except for when a room is added or removed. The data is NOT saved (that would require a change in the map file format!) and NOT regenerated when a map is loaded. The consequence is clear - rooms that had exits TO a room being deleted are not having the exit cleared, and calling up the room exit dialog is just one place where this WILL cause a crash.

I will look to fix the underlying issue to (re)generated this data on loading and also make the dlgRoomExits class more robust.

As this causes a terminal crash on the current release 3.0 preview (delta) I think it merits elevation to "Critical" status. 8-(

Yeah okay. What what issue fixed was super-slow room deletion, it took
forever to delete rooms before it.

Pull Requests up for testing / review for both development https://github.com/Mudlet/Mudlet/pull/247 and release_30 https://github.com/Mudlet/Mudlet/pull/246 branches.

Fixes committed to release_30 branch by https://github.com/Mudlet/Mudlet/pull/250 and https://github.com/Mudlet/Mudlet/pull/255, back-port of commits to development branch still to be done...