HaikuDepot: Some usage of the BKeyStore API

... completely untested and premature. The idea is that after a successful
login with the web-app, the password used is stored in the keyring. Then
HaikuDepot will restore just last the used username on next launch and retrieve
the password from the keyring. One could also register multiple accounts in
HaikuDepot and switch between them.
HaikuDepot: Force preferred language to be supported

At the moment, the web-app has a defined, limited set of supported languages
for translations. JSON requests fail when asking for an unsupported language
code. For the time being, force the language code to be in the supported set.