edje: Add dpi feature.

edje: Add dpi feature.

Summary:
This dpi is used to get the scale for each collection.
If each collection has a described dpi, it calculates a proper scale
based on the dpi and dpi which is described in the collection.

@feature

Test Plan:
If add dpi to collection of edc, the edje will save the value as the dpi of the collection.
For example, if the dpi of your device is 100, you just set dpi: 100 in the collection of edc.
If the edj is loaded in another device(dpi is 200), it will scaled 2 times.
It is possible that the described dpi of application and theme are different.
In that case, application and theme have a different scale.
It makes the edj that made in different environment works in one device.

Reviewers: seoz, zmike, JackDanielZ, Hermet, woohyun, cedric, raster

Reviewed By: raster

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D1190