Use QSharedPointer for cross-thread Link references

Also many LinkManager API changes to further isolate all link
management inside LinkManager.

Bump to 2.4 Developer Release

Update README.md

Switch to Stable V2.3 Release Candidates

Cleaning up Tile Cache Queue

Update Ubuntu required packages

Advanced Power Config

Fixing list of items in Quick Info View

UPD Link Fix

Remove old Flight Mode config code

PrimaryFlightDisplay: Clear up speed label (now showing airspeed and gro...

Light palette color changes

Update Ubuntu PPA instructions

Made light palette reverse of dark palette

Crosstrack error

Line chart widget: Auto-force ground time if offset is more than 3 seconds

Forgot to remove controller rotation setting

Disable calibration buttons while calibrating

Line chart widget: Auto-force ground time if offset is more than 3 seconds (30% of default view width)

Also changed to pull specific small parameters set after cal instead of
full refresh. This way, the red/green indicators update much more
quickly.

Preflight check