Explicitly send events for modifier keys in wxUIActionSimulator.

Explicitly send events for modifier keys in wxUIActionSimulator.

Key down/up events for the modifiers were already explicitly sent under wxMSW
and wxOSX but not under wxGTK where, as the result, the corresponding events
were not generated at all.

Do send these events explicitly to make the events generation consistent under
all platforms now. This means that wxUIActionSimulator::DoKey() now generates
exactly one event everywhere.

Notice that the modifiers for the key events generated by the modifier keys
are also the same under all platforms now which is not the case for the events
actually generated by the user (wxMSW sets the corresponding bit for the
modifier key down event but not the key up one while wxGTK does exactly the
contrary). This should be fixed in the future so that wxUIActionSimulator
generates the same sequence of events as the user would and that it's still
the same for all platforms.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65516 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775