Custom plugin work

Made QGCCorePlugin derive from QGCToolBox and expose everything through the same mechanism as everything else.

Bad reference fixes

Allow changes of log saving if not armed with reboot

Plugin support

Allows QGC plugins to be optional or custom

From #4256 

> This allows you to create custom builds of QGC which may or may not contain all of the plugins. It also allows you to create a custom build where you can re-use the PX4 or APM plugin code as the base class for you own custom vehicle plugin and then expose only that to the system.

fix for MAVLinkLogManager

Per-Vehicle parameter load progess

Also removed intermediate param load retry message

Open file and directory dialogs in foreground

ParameterManager: Better logging, plus some retry fixes

Fix combo selection bug

Meta data for ArduPlane 3.7

OS X Installer: Bump image size

Swipe view click

Small fixes

Don't change active vehicle for additional vehicles

RallyPoint support should be optional

The return lead to data being dropped. Some of this data could have been
part of the ulog header, which lead to corrupt files.

One heartbeat per-link, not per-vehicle