Fix floor_location (Floor location) is not persisted between boots of OSC

[adamkewley]

• Broken out from Remember muscle styling options #782
• Depends on vec3 support in AppSettings, which is handled by Add glm::vec3 to AppSettings #787

Changing the location (principally, the height) of the 3D viewport's floor is not persisted between boots of OSC. The reason this is the case is because the settings classes cannot persist glm::vec3. Similar configuration support would be handy for (e.g. default camera locations).