[Custom handler] Content listener

[Custom handler] Content listener

[Issue#] N/A
[Feature] Handle content (mime type) from custom handlers.
[Problem] N/A
[Cause] N/A
[Solution] Added content (mime) listener. It will check if widget
was run by appservice and will look for proper page to show.
[ScmRequest] wrt-commons
This patch depends on:
https://tizendev.org/gerrit/#/c/20791/

[Verification] Run manual test from wrt-extra:
1. Run sh registered_handler_register.sh to add widget to appsvc and to custom handlers.
2. Install register_handler.wgt
3. Run wrt-appsvc-test
4. Verify that custom.html from register_handler.wgt was run with proper file in uri.

Change-Id: Ib0434a9f3409896681e7a536eaed4ad42af0bb7c