Fix incorrect event handler functions casts in wxRibbonPanel code.

Use wxMouseEventHandler() instead of (wrongly) casting a method taking
wxMouseEvent to wxEvent-taking wxObjectEventFunction.

Closes #15550.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74914 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Add wxActivateEvent::GetActivationReason().

This method is implemented for wxMSW-only currently and allows to check
whether the window is being activated by a mouse click or in some other way
there.

Closes #15516.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74915 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775