OSX: implement QFontMetrics::maxWidth().

CoreText doesn't seem to provide us with a "maximum advance" value,
but 0 is really wrong, it leads to QLineEdit::minimumSizeHint() being
0 since it's based on maxWidth(). It even led to a negative min width
with setTextMargins(-1, 0, -1, 0).

Change-Id: I4faf8ecfb6d91e9dff66ec63651d003014503cb4
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
tst_qtableview: fix virtual-override clang warning

Change-Id: I40391890c988ce15d8fa65898976bf0fdc446992
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
QAbstractProxyModel: fix canDropMimeData/dropMimeData implementations

The code in 4696e9dbaa4 was incorrect. It is perfectly valid to call
these methods with row=-1 column=1 parent=some_index, this is exactly
what happens in QListView and QTableView. Child row/column is only for
trees.

Move the coordinate mapping from QSortFilterProxyModel into a new
mapDropCoordinatesToSource internal method, used by QAbstractProxyModel.

Task-number: QTBUG-39549
Change-Id: I3312210473d84b639cbe4c01f70ea36437db3e91
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Stephen Kelly <steveire@gmail.com>
QAbstractItemView: call canDropMimeData, as one would expect.

The virtual method was added for 5.0 but never called.

The old code (only checking mimetypes) is now the default implementation
for canDropMimeData. Model subclasses can now refine this by having
index-specific logic instead, or in order to inspect the dropped data
(e.g. to accept files and refuse directories, which are all text/uri-list).

[ChangeLog][QtWidgets][QAbstractItemView] now calls canDropMimeData in
order to decide whether or not to accept the drop.

Task-number: QTBUG-30534
Change-Id: Ied3aa964b4025bae6a1a26df89a681bfe61c3faa
Reviewed-by: Stephen Kelly <steveire@gmail.com>
QAbstractItemView: rename private canDecode to canDrop.

Change-Id: I9b1b3ead0bea35a75e20c5c18e288251de7ad7fe
Reviewed-by: Stephen Kelly <steveire@gmail.com>