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(cherry picked from commit 4fefa445)

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 dafb63fd)

(cherry picked from commit 17270bb3)

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 b74c33db)

:P

(cherry picked from commit 66263604)

(cherry picked from commit 5a077649)

Gah :)
(cherry picked from commit 49375a3d)

(cherry picked from commit baa8aadc)

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 fa9161e4)