custom widget: ignore unpaired 'mouse up' event

custom widget: ignore unpaired 'mouse up' event

- bug: if select file with double click from file dialog, unexpected button will be clicked of emulator manager
-- after double click event, mouse up event occurred in windows
-- custom widget raise selection event when received mouse up event
-- ubuntu, mac os does not happen

- fix: match mouse down event with mouse up event
-- if does not receive mouse down event, mouse up event will be thrown out

Change-Id: Ie7b1f5f5feac227cfc0f3d62fc179d748e5ec14c
Signed-off-by: jihye424.kim <jihye424.kim@samsung.com>