Delay GtkIMContext calls until widget is realized.
This may avoid a locking bug seen on Ubuntu which causes a deadlock.
See #16202


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76398 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775