Added MainPanel actions n and N for find next and find prev

[syrrim]

this was discussed in issue #601:

• I didn't add an action for P because this has since been used

• I don't think concerns regarding overlap with top commands are overly important, because the relevant top commands haven't been added in the meanwhile

Besides that, I think that these are very standard key combos, being used in a variety of unix tools as well as less.

[natoscott]

Merged here: htop-dev/htop@46ab1aa