Define plot icon style per map entry

[AHeroicLlama]

https://discord.com/channels/1029499482028646400/1029512633319170133/1233548466169577512

Current plot icon behaviour: Icon colours and shapes are predefined, and adding increasing amounts of plots cycles through these. This is a very simple approach for users who don't care about appearance. However it is not practical to amend the style of icons for any given plotted item.

Desired behaviour: It should be possible to select items in the 'items to plot' and amend their style individually. It is assumed a default unique style is preselected until overridden by the user.