command: mark some commands as "scalable"

Scalable commands (seek, cycle and add) understand the cmd->scale
parameter and will "scale" their action accordingly, for example, a seek
with scale = 0.5 will only seek half the specified amount and a seek
with scale = 2.0 will seek twice as much.

Mark these commands so in the next commit, input.c will be able to
synthesize input with cmd->scale = 1 for non-scalable commands.