issue1622: metricsview kerning, charview multichar trailing /.

- allow trailing slash in charview
- don't crash when altering kerning in metricsview and there are multiple kern pairs visible with the current value
- mv: update other kern pairs when changing the current kern pair.

cc #1622.
this cleans up dangling references to kp. Before this PR if you had a kerning of -10 and kept moving it towards 0 then you could at 0 get a dangling reference to a kp which has been freed.