Fix #8452 (app's crashing for non-existent system catalog)

* make the system catalog a BCatalog instead of a BCatalogAddOn*,
such that using a non-existing system catalog won't crash but
simply return the untranslated string instead
* rename MutableLocaleRoster::GetSystemCatalog() to LoadSystemCatalog()
and adjust it to use BCatalog::SetTo() in order to replace the
data used by the given catalog
* adjust all users of gSystemCatalog accordingly

Merge liblocale.so into libbe.so.

* Make the locale kit a part of libbe.
* Drop the LocaleBackend kludge used from within libbe (and from
other places, too) in order to access system catalog strings.
This is now done via gSystemCatalog, which is provided and initialized
by libbe.
* Drop all references to liblocale.so from all Jamfiles.
* Add legacy symlink liblocale.so in order to keep optional packages
that rely on it in a working state.

TODO: the documentation hasn't been updated.

Fix #8452 (app's crashing for non-existent system catalog)

* make the system catalog a BCatalog instead of a BCatalogAddOn*,
such that using a non-existing system catalog won't crash but
simply return the untranslated string instead
* rename MutableLocaleRoster::GetSystemCatalog() to LoadSystemCatalog()
and adjust it to use BCatalog::SetTo() in order to replace the
data used by the given catalog
* adjust all users of gSystemCatalog accordingly

Merge liblocale.so into libbe.so.

* Make the locale kit a part of libbe.
* Drop the LocaleBackend kludge used from within libbe (and from
other places, too) in order to access system catalog strings.
This is now done via gSystemCatalog, which is provided and initialized
by libbe.
* Drop all references to liblocale.so from all Jamfiles.
* Add legacy symlink liblocale.so in order to keep optional packages
that rely on it in a working state.

TODO: the documentation hasn't been updated.