Optionally color nicknames in all messages

Find senderHash for all nickname-relevant messages that don't have
multiple nicknames (e.g. netsplits).

Add new preference "UseNickGeneralColors" to toggle sender coloring
on all available nicknames, not just action messages.  This might be
undesired by some given the level of colors, so it's kept as a
separate preference from ACTION message coloring.

Bump settings minor version to keep nickname coloring disabled for
existing setups.