Merge atk and at-spi2-atk repositories into at-spi2-core

This is The Great Merge that I have been talking about. The idea is that by merging the at-spi2-core, atk, and at-spi2-atk repositories into a single one, we can write end-to-end or integration tests more easily and refactor all three modules quickly.

• Prepare atk repository for merging, in this personal branch
• Prepare at-spi2-atk repository for merging this personal branch
• Merge atk's history into at-spi2-core
• Merge at-spi2-atk's history into at-spi2-core
• Hook atk/ library sources into the build
• Hook tests/atk into the build
• Hook docs/atk into the build
• Add CI job for building the reference docs
• Add CI job for deploying the reference docs to gitlab pages (done, not tested)
• Hook droute sources from at-spi2-atk into the build
• Hook atk-adaptor sources from at-spi2-atk into the build
• Hook tests/at-spi2-atk into the build
• Merge atk-po translations into the main po directory
• Link to libatspi reference docs from README.md
• Link to atk reference docs from README.md
• Add maintainer info from atk.doap and at-spi2-atk.doap into at-spi2-core.doap
• Add a devel-docs/roadmap.md document.

After the repo merge:

• Tasks moved to #54

CC @mgorse @ebassi @joanmarie @sthibaul

Notes on the merge:

• I have not included the part in at-spi2-atk that made it possible to use atk's deprecated functions without warnings. I do want to fix the use of those deprecations in at-spi2-atk, and just leave the old functions available in atk for ABI compatibility with existing apps.