Don't blindly append to the end of the list, but try to put files into
the right place with regards to lexicographical order.
We do not re-sort existing file lists, nor do we attempt to be fancy
with strangely formatted project files. But lists conforming to our own
style will stay sorted, in particular lists that have only ever been
touched by Qt Creator.
Fixes: QTCREATORBUG-553
Fixes: QTCREATORBUG-21807
Change-Id: I15db0e867a2d477fcf95e008f98ba468dcd83e45
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
a420374976