GCS: Refactor Uploader

[tracernz]

Backend stuff and upgrader needs to move out of UAVGadget as there can be many instances of this. The gadget should be simply a UI displaying the current state of the backend, and allowing the user to send commands to the backend when appropriate. The plugin should keep track of state etc. The plugin should provide some additional public functions such as reboot to ConfigTaskWidget and DefaultHwSettings, and active to SetupWizard (to block auto-launch of it).