Add a "markers" page to the performance-log viewer, which lists
the event markers contained in the log, and allows navigating
between them.
Update docs accordingly.
(cherry picked from commit dafb63fd66)
In the perofmance-log viewer, add header-bar buttons to clear and
invert the selection, and allow inverting the selection by ctrl-
right-clicking on the sample-selection area.
Update the docs.
(cherry picked from commit b74c33db5c)
Add devel-docs/performance-logs/performance-logs.md, which
describes how to record and view performance logs, and how to
report perofrmance-related issues.
(cherry picked from commit fa9161e4f2)
