Improve usability of activity list

People often struggle with activity list when the list is quite long.
Example of issues:
* scrolling in the list to locate something specific can be hard when the list is long
* finding a warning is not always easy unless the top of the list is visible
* finding a Talk or other kind of notifications is not always quick or easy

Should we really keep many entries ?
Currently when the client is propagating changes, they will each get an entry such that the list would display the changes in case the activity server app is not enabled.

Start by locating the desktop client's activity-list implementation and tracing how propagated changes are added as entries. Review the existing activity behavior and the reported difficulties with long lists, warnings, and Talk notifications. Done requires a clearly agreed usability design and corresponding implementation, but the issue does not identify files or tests.

Written by the indexing model from the issue text.