entry: add elm_entry_select_allow_set/get APIs

entry: add elm_entry_select_allow_set/get APIs

Summary:
There is no way to allow/deny the text selection feature.
It is only controlled by disabled state. But, some UX does
not want to allow the text selection on editable entry widget.
@feature

Test Plan:
Run the following test case. You can see "Select Allow" check box.
elementary_test -to entry

Reviewers: tasn, herdsman, cedric, thiepha

Reviewed By: thiepha

Subscribers: jpeg

Differential Revision: https://phab.enlightenment.org/D3934

Change-Id: I433ec3b894033b88a4be1364bab6663d3d187a57