Fix crash when removing a notification while we are adding one

Reading https://bugs.kde.org/show_bug.cgi?id=311871#c41 you can see that it happens that we are doing a
78 0x00007f4eff8c5ffb in QDeclarativeListModel::insert (this=0x1ebbdb0, index=0, valuemap=...) at util/qdeclarativelistmodel.cpp:436
and then we end up reentring and doing
16 0x00007f4eff8c737f in QDeclarativeListModel::remove (this=0x1ebbdb0, index=6) at util/qdeclarativelistmodel.cpp:402

Some of the stuff that depends on the QDeclarativeListModel doesn't seem to like getting a "remove" while a "insert" is happening and to be honest m in no mood to fix that, so basically I'm protecting against that happening in our QML code. From what i read you have to be extremely unlucky since the timer  only triggers each 10 minutes and it has to trigger at the same time a notification is being added, but oh well, the backtrace points to it and two different people in two different systems say it has stopped the crashes so I don't think it hurts to have this in.

Acked by Aaron J. Seigo
REVIEW: 113471
BUG: 311871
FIXED-IN: 4.11.3