Text selection refactoring

Text selection refactoring

This Task contains two main parts:

1- Introduce internal interface Dali::Toolkit::Text::SelectableControlInterface which is inherited by TextEditor and TextField
Both Class needs to implement this interface methods, else developer will get compilation error

2- Introduce Properties to get/set Text Selection Ranges start and end positions in Devel namespace, SELECTED_TEXT_START (start position) SELECTED_TEXT_END (end position)
this allow programatical set for text selection, also get selection range programatically

Change-Id: I7ec96d4f1570218f0e945f21a03a6a10e5e8fe97