Allow specifying the target collection/wallet

[michaelk83]

Summary

It should be possible to override the target collection or wallet on backends where this is possible, using a new pair of methods Job::setCollection() and Job::getCollection(). On backends where this is not possible, the setting would be ignored. If not set, default to the collection or wallet that is used currently.

Context

This is needed for client applications that may want to store their passwords separately from the rest.
See KDE bugs 228308 and 172523.

Suggested Implementation

• With libsecret: if set, pass the set collection name to secret_password_store_fn(), otherwise pass SECRET_COLLECTION_DEFAULT as now.
• With KWallet: if set, call open() with the set name, otherwise call networkWallet() first (as now) to determine the default wallet name.

[michaelk83]

It could also be useful to have an environment variable for this, so that the end user can force a client application to use a specific collection of their choice (inspired by keepassxreboot/keepassxc#8517 (comment) ).