Applet to popup when a USB device is connected

 This is part of a suite of python3 and Vala based applets for the Budgie
 Desktop that provide additional user orientated capabilities.
 The applets can be used individually or as a set.
 .
 The DropBy applet pops up in the panel when connecting a usb device.
 The applet subsequently offers the option(s) to mount, unmount/eject
 and in case of a flash drive, to make a local copy of the drive's
 content. The info shows the free space on the volume.
 .
 The applet is context- sensitive, meaning: it only shows its icon in
 the panel if, and as long as, any usb drive is connected.