Release 1.0.0-alpha.1

Use SemVer as our versioning scheme, and release libadwaita 1.0.0-alpha.1 as the first alpha version of the libadwaita 1 API. The first beta will be 1.0.0-beta.1 and the first stable will be 1.0.0.

Given the library was unreleased and pre-alpha, this version number change isn't regarded as a break but as a fix, and applications using the library and checking its version number are held responsible for updating their code.

Fixes #44 (closed)