Using QFileDialog directly on OS X can hang intermittently due to Qt
bug.

- Logging now always on to temp file
- User prompted to save temp file after disconnect
- Orphaned temp logs checked for on boot
- Logs saved to new save file location
- Log file starts/stops with first and last connnection

Centralize all connecting and disconnecting of links through
LinkManager::[Connect|Disconnect]Link api. Connect/Disconnect no longer
allowed through LinkInterface. This allows creation of new
LinkManager::setConnections[Suspended|Allowed] methods to disallow
connections as needed.

This is the Pixhawk research board, not the PX4 production board.

This removes double emit of signals.

This api doesn’t make any sense given the fact that LinkInterface
signals bytesReceived with the bytes in the signal. Also the reason why
it is never called!

This reverts commit b4089ad9.

This way we can test with QT_FAIL_WARNINGS set.

Found by valgrind.

SLUGS has maintained their patches out-of-tree for a long while now.
These will eventually get merged back in, but it'll be easier to evaluate them as fresh commits instead of modifications to this old & dead code, so we'll delete this for now.