Add wxWithImages helper mix-in with {Set,Get,Assign}ImageList() methods.

Avoid defining SetImageList() in several different places in wx API as not
only this resulted in (trivial) code duplication but this method also had
different semantics before: it didn't take ownership of the pointer passed to
it in wxTreeCtrl, wxListCtrl and wxBookCtrl and derived classes but did take
its ownership in wxDataViewTreeCtrl and wxRichTextFormattingDialog.

Harmonize this for all the classes now: SetImageList() never takes ownership
while AssignImageList() (which is now available in all classes having
SetImageList()) always does.

Also add convenience wxWithImages::GetImage() helper to avoid (more) code
duplication in wxDataViewTreeCtrl code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68809 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/dataview/dataview.cpp

@@ -684,7 +684,7 @@ void MyFrame::BuildDataViewCtrl(wxPanel* parent, unsigned int nPanel, unsigned l
 
             wxImageList *ilist = new wxImageList( 16, 16 );
             ilist->Add( wxIcon(wx_small_xpm) );
-            tc->SetImageList( ilist );
+            tc->AssignImageList( ilist );
 
             wxDataViewItem parent =
                 tc->AppendContainer( wxDataViewItem(0), "The Root", 0 );