Pass recipient ids, not recipient objects

Fixes #2233
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Include group titles in search

When searching for messages only simple threads matching the
contact names are returned as search results. With this commit
also group converstations where the group title matches the search
term are displayed in the result. This makes search results more
consistent with the conversation list as now all conversation
titles (i.e. contact names and group titles) are searched through.

Fixes #1954
Closes #2216
More l10n - for decrypting errors

//FREEBIE

Closes #2231
thumbnail generation and disk caching

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