This will improve maintainability.

This is a follow up of #842.

• merge de-g1-bidi-core.cti with de-g1-core.cti (done in Integrate regular and "bidi" German tables #1100)
• merge de-g0-bidi-core.uti with de-g0-core.uti (done in Integrate regular and "bidi" German tables #1100)
• also apply de-g0-bidi-specs.yaml tests on de-g0.utb in order to explain the differences between the tables (moved to Refactor the German test files #1175)
• same for de-g1-bidi-specs.yaml (moved to Refactor the German test files #1175)

Also:

• consider changing the terms "bidirectional" and "back-translatable" in the metadata to "detailed", and also in the file names (done in Integrate regular and "bidi" German tables #1100)