textctrl.cpp: Removed assert from CanRedo/CanUndo since the best behaviour is

textctrl.cpp: Removed assert from CanRedo/CanUndo since the best behaviour is
to return FALSE (Redo/Undo functions are unimplemented)
treectrlg.cpp: in InsertItem, allow previous item to be null in order
to insert at first position of possibly-empty child list
dcclient.cpp: allow setting of null font (common espec. on Windows)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@15540 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775