[Indicators] Add support for ayatana indicators

[mariogrip]

This adds support for ayatana indicators but keeps the existing unity indicators to support legacy indicators, this also cleans up the indicator load logic.

This opts to add each indicator menu types to the qml object instead of doing a hacky replace string logic, this way we save cpu cycles but at a ultra minor memory bump.

[peat-psuwit]

LGTM, but someone else must approve. Didn't test or anything.

As this merges to xenial branch, maybe wait for OTA-13 to be released first?

[mariogrip]

@UniversalSuperBox what do you think?

[dobey]

Needs to update/add tests and data for these changes.