Add wxControl::GetSizeFromTextSize() to size the control to its text.

This function can be used to size a, say, wxTextCtrl to be exactly of the size
needed to enter the given amount of text in it.

This patch adds wxGTK implementation for wxTextCtrl, wxChoice and wxCombobox;
changes to the samples and the documentation.

Closes #14812.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72935 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/combobox.h

@@ -145,6 +145,8 @@ protected:
     // custom list stores.
     virtual void GTKCreateComboBoxWidget();
 
+    virtual wxSize DoGetSizeFromTextSize(int xlen, int ylen = -1) const;
+
     virtual GtkEntry *GetEntry() const
         { return m_entry; }