Provide way to show a variable chooser widget.

For the Qt Creator variables, currently used in the external tools.
The variable chooser looks if the current focus widget has a variable
support property set and is a line edit, text edit or plain text edit.
For line edits it adds a little icon button that shows the chooser.

src/plugins/coreplugin/coreconstants.h

@@ -261,6 +261,8 @@ const char * const SETTINGS_DEFAULTTEXTENCODING = "General/DefaultFileEncoding";
 
 const char * const ALL_FILES_FILTER      = QT_TRANSLATE_NOOP("Core", "All Files (*)");
 
+const char * const VARIABLE_SUPPORT_PROPERTY = "QtCreator.VariableSupport";
+
 const int TARGET_ICON_SIZE = 32;
 
 } // namespace Constants