Move selection code into content/ module.

Move selection code into content/ module.

Currently, selection code (SelectionBoxEfl, SelectionControllerEfl,
SelectionHandleEfl and SelectionMagnifierEfl) are into ui/base
directory.

But those objects are used in content and are even defined in namespace
content. So those files definitely belong in content/ module.

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=13303
Reviewed by: Antonio Gomes, SeungSeop Park

Change-Id: Iae3f7e76978223cec4d5925c915d0fb529e1290c
Signed-off-by: Arnaud Renevier <a.renevier@samsung.com>