QML Performance: MissionItemEditor creation expensive

[NaterGator]

I've been doing some profiling of master and I am finding that the MissionItemEditor component is particularly expensive for QML to create. This problem becomes apparent when connecting to a vehicle with a survey mission onboard (or downloading a survey mission) where a bunch of mission items are included in the plan.

It obviously isn't critical, just something we should address in a future release. I think a good approach is to either defer the full underlying item editor (the editorLoader accounts for ~55% of creation time) or better yet to have incredibly simple delegates that move a singleton full MissionItemEditor into their component and bind it to their underlying mission item.

[NaterGator]

Sigh. Too many windows open, moving this over to qgc.