Merge goldendict-help repository into goldendict repository

[vedgy]

Compiling the help databases should be part of a regular GoldenDict build, similarly to how translations are currently compiled.

Before merging would have to convert goldendict-help line endings from Windows to Unix style, as in the goldendict repository.

Advantages of the merge:

1. The documentation will be extended in a commit that implements a new feature, not in a separate goldendict-help commit.
2. Being in the main goldendict repository may attract documentation translators.
3. Less developer work: no more need to update the documentation DB binaries in the goldendict repository.
4. Eventually smaller goldendict git repository size thanks to the absence of the binaries.

Questions:

1. Is goldendict-help a separate repository only because the Qt 4 documentation syntax is incompatible with Qt 5?
2. Is there no usable common Qt 4 and Qt 5 syntax?

If there is no compatible Qt 4 and Qt 5 syntax, the merge would have to wait until Qt 4 support is dropped (one more reason to drop it sooner). Hopefully a common Qt 5 and Qt 6 documentation syntax does exist.