Fix crashes in KFileItemModel::removeItems()

These crashes were caused by the recent commit
ff3267c6dcd0f6d0621b3a96b5462a9581a03883. It introduced two problems:

a) A logic error in the code that removes the ItemData pointers from
m_itemData that could cause crashes if multiple item ranges are
removed, and there were un-removed items behind the last one.
b) The implicit assumption that any call of removeItems() will actually
result in items being removed in the model. This is incorrect if
the model is first cleared and then the hidden-files setting is
modified, which happens if "Save view properties for each folder" is
enabled, and a folder where hidden files are shown is left. In that
case, the dir lister emits itemsDeleted for the hidden items after
they have been removed from the model due to the folder change.

I'll add a unit test covering these issues soon.

Many thanks to Romário Rios and Hrvoje Senjan for testing!