WML Action Candidates for Mainline

Definitely:

• [hidden_unit] → [unit] hidden=<boolean>
• [animate_attack] → extend [harm_unit] to allow simultaneous attacker+defender animations
• [remove_sound_source] supporting comma-delimited id list

Maybe:

• optimized versions of mainline action macros ([quake], [fade_in], [fade_to_black])
• [transient_message]
• [show_image] (see also mainline bug #10519)