Unified search: indicate that more providers are loading

[tobiasKaminsky]

• use unified search
• 
• see above "conversations" as last provider
• one does not know that other, e.g. files search provider is still queried

Suggestion, found together with @AlvaroBrey, is to show in footer:
"Search results for files, messages are pending".

This is then changed once there is a result of those providers: ether show provider's result or update footer.
In the end footer is then not shown.

@jancborchardt

[jancborchardt]

Search results for files, messages are pending

Less technical: "Searching files, messages and events …" with a spinner left next to it.