Enable viewing bracketed user events on a per-PE basis

[stwhite91]

Original issue: https://charm.cs.illinois.edu/redmine/issues/1901

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When using Projections to visualize traces with AMPI and a high degrees of virtualization (>8 ranks/core), displaying all of the bracketed events on all PEs takes up too much space to be practical. It would be nice to be able to click on a PE and choose to view the bracketed events on that PE, or to be able to select a range of PEs to display them on.