v0.3.0-preview

Changes since the last preview build:

• Possible fix for the missing marker texture reported on Discord
• Fix #1: reset the window to default values, when the window is opened with an invalid target (e.g. a vessel that doesn't exist in the current scene)
• Fix an oversight in the implementation of 1c2bdec: disable vessel selection when switching from "Origin" to "Destination" in the body selection window.

Full Changelog: v0.2.0-preview...v0.3.0-preview