... in backtraces

In the performance-log viewer's backtrace viewer, show a document
icon next to stack frames with source-location information, whose
source file is found locally.  Clicking the icon opens the source
file in a text editor at the relevant line.

Two environment variables control this feature:

  - PERFORMANCE_LOG_VIEWER_PATH is a list of colon-separated
    directories in which to look for source files.  If this
    variable is undefined, the current directory is used.

  - PERFORMANCE_LOG_VIEWER_EDITOR is the command to use to launch
    the text editor, for editing a specific file at a specific
    line.  The special strings "{file}" and "{line}" are replaced
    with the filename and line-number, respectively.  If this
    variable is undefined, "xdg-open {file}" is used.

(cherry picked from commit 0f387092)