edje entry: improve selection performance

edje entry: improve selection performance

Summary:
The selection performance is slow if we select large chunk of text.
This is caused by many rectangles created and deleted.
This patch provides a way to improve it by combine selection rectangles
of line in middle into one rectangles (i.e, if we have N lines,
the selection rectangle for lines 2 to N-1 will be combined into one.)

@feature

Reviewers: raster, cedric, tasn

Subscribers: herdsman, woohyun, cedric

Differential Revision: https://phab.enlightenment.org/D1508