Provide a list for selecting peeps

[SunsetFi]

This PR adds a peep selector list option to the Peep Editor.

When peeps are too close together on one tile, selecting them with the piper tool becomes impossible. While picking up and putting down a peep elsewhere can be workable, it may cause changes in the scenario random seed, which for complicated reasons may not be allowable.

By adding a list, it becomes possible to browse and select peeps regardless of their map position.

Features:

• Lists all guests and staff
• Shows entity number and staff type
• Selects peeps on click
• Highlights the currently selected peep in the list
  • Note: The list will not re-render correctly for this in OpenRCT v0.4.32 - I submitted a fix here

Currently, the list is not sortable, as the usage of list item separators causes a crash on sort in OpenRCT. As an alternative to this, the window could swap between a guest-mode and staff-mode, removing the separators and allowing the list to be sorted.