Setting version to 0.15.90 in preparation for 0.16.0 release

For now on, no new API or ABI changes are accepted. Just fixes
on existing features.

Fixed politic for ABI versioning and sync Autotools and Meson
to follow it.

0.16 may will be next stable ABI/API in front of 1.0 release.

Consider that any Interface no implemented into 0.16 series and
no updated to follow's Vala/C API conventions, is suitable to change
its name for methods and properties, some times their accessors'
type, in order to provide an stable API/ABI for 1.0 release.