QtMacExtras: Add APIs to Set the ToolTip of a Button

Currently, QtMacExtras does not support setting the tooltips of the buttons.

I am attaching a patch that implements this functionality:
—
src/macextras/qmacnativetoolbar.mm | 4 ++--
src/macextras/qmacnativetoolbar_p.h | 2 +-
src/macextras/qmactoolbardelegate.mm | 11 +++++++++++
src/macextras/qmactoolbardelegate_p.h | 1 +
src/macextras/qmactoolbutton.mm | 10 ++++++++++
src/macextras/qmactoolbutton_p.h | 4 ++++

In particular, QMacToolButton stores the tooltip. QMacNativeToolBar's addAction(QIcon, QString) has gained a new parameter for the toolTip (initialized with an empty string) [1]. QMacToolbarDelegate's toolbar: method (invoked by Cocoa) checks whether a button has a tooltip, and if so, sets the toolTip using the NSToolBarItem's setToolTip: method.

[1] You may want to add a new override instead of a default-initialized parameter. Not sure what the Qt-preferred way is.