Remove `explain-pause-profiles` and integrate into `explain-pause-top`

[lastquestion]

Add the buttons for slow runs into the rows in explain-pause-top. Remove the explain-pause-profiles hard-coded machinery; add a new hook similar to explain-pause-measured-command-hook, probably explain-pause-profiled-command-hook, that fires when a profile is saved. Thus, separate the core measurement engine with the display layer.

This should clear the way to fix #16, too.