Use IME in wxGTK wxComboBox too

There was confusing with the method GTKIMFilterKeypress() in wxGTK
wxTextEntry: it was called the same as wxWindow virtual method of the
same name, but didn't override it, of course, as wxTextEntry doesn't
derive from wxWindow.

It also wasn't called for wxComboBox which inherited from wxTextEntry
but didn't override wxWindow::GTKIMFilterKeypress() to actually call its
method.

Fix this and rename the wxTextEntry to use a different name for clarity.

include/wx/gtk/textentry.h

@@ -91,8 +91,9 @@ protected:
     virtual bool DoAutoCompleteStrings(const wxArrayString& choices) wxOVERRIDE;
     virtual bool DoAutoCompleteCustom(wxTextCompleter *completer) wxOVERRIDE;
 
-    // Override the base class method to use GtkEntry IM context.
-    virtual int GTKIMFilterKeypress(GdkEventKey* event) const;
+    // Call this from the overridden wxWindow::GTKIMFilterKeypress() to use
+    // GtkEntry IM context.
+    int GTKEntryIMFilterKeypress(GdkEventKey* event) const;
 
     static int GTKGetEntryTextLength(GtkEntry* entry);