Add mainloop interface and backend layer

Add mainloop interface and backend layer

popup service needs to use ecore loop based mainloop so as to expand
ecore main loop life cycle to process life cycle to fix restart ecore
loop bug.

Make Mainloop class abstract and declare Event which can converted to
backend specific event enums(e.g., epoll events, ecore events) then
mainloop and service class can be defined generically.

Mainloop backend is selected in each service impl's constructor.

Change-Id: Ie965bbcca45c31a950cbb86649a2d22dd0d494d1
Signed-off-by: Kyungwook Tak <k.tak@samsung.com>