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/control.h

@@ -12,6 +12,7 @@
 
 typedef struct _GtkLabel GtkLabel;
 typedef struct _GtkFrame GtkFrame;
+typedef struct _GtkEntry GtkEntry;
 
 //-----------------------------------------------------------------------------
 // wxControl
@@ -100,6 +101,12 @@ protected:
     // Fix sensitivity due to bug in GTK+ < 2.14
     void GTKFixSensitivity(bool onlyIfUnderMouse = true);
 
+    // Ask GTK+ for preferred size. Use it after setting the font.
+    wxSize GTKGetPreferredSize(GtkWidget* widget) const;
+
+    // Inner margins in a GtkEntry
+    wxPoint GTKGetEntryMargins(GtkEntry* entry) const;
+
 private:
     DECLARE_DYNAMIC_CLASS(wxControl)
 };