combobox: add Multiple selection feature

combobox: add Multiple selection feature

Summary:
This patch focuses on Combobox widget customization,
Multibuttonentry widget is used instead of entry for taking user input.
The idea is to make the widget look like  {F28112} {F28115} when the multiple_selection is set.
To-DO:
1) Need to add scrollable interface to combobox when MBE is used (need some suggestions on it).
2) focus cycle is still buggy as genlist requires focus otherwise selected item will return NULL (sometimes)
Signed-off-by: divyesh purohit <div.purohit@samsung.com>
@feature

Test Plan: Please run combobox multiple selection example from elementart_test.

Reviewers: raster, shilpasingh, cedric

Subscribers: govi, rajeshps

Projects: #elementary

Differential Revision: https://phab.enlightenment.org/D3570