Lib/Indicator: Add enum for indicator position

[teamcons]

Fixes https://github.com/elementary/wingpanel/issues/608

Add a property for indicators for initial placement. For Launcher and Datetime, an ugly workaround give them the correct positioning until the property is declared in their respective repos.

i am not sure where to put the enum definition where it doesnt get lost in code. Having its own file for three lines feels overkills.

• [x] Test this
• [ ] More elegant ugly workaround

[teamcons]

Builds fail because of this but it is not mine