Cinnamon: Request: Option to fix desklet location relative to particular display

```

• Cinnamon 3.6.7
• Mint 18.3
• Graphics: Card-1: Intel Xeon E3-1200 v3/4th Gen Core Processor Integrated Graphics Controller
  bus-ID: 00:02.0 chip-ID: 8086:0412
  Card-2: NVIDIA GM206 [GeForce GTX 960] bus-ID: 01:00.0 chip-ID: 10de:1401
  Display Server: X.Org 1.18.4 driver: nvidia Resolution: [email protected], [email protected]
  GLX Renderer: GeForce GTX 960/PCIe/SSE2 GLX Version: 4.6.0 NVIDIA 390.30 Direct Rendering: Yes

• 64 bit
  ```

Issue
Desklet location is mapped to absolute coordinates of the entire desktop. This causes the desklets to change location if monitors are added/removed.

Steps to reproduce
On a multi-monitor system, move (virtually) one monitor over the primary monitor. Then disable all the monitors, except the primary.

Place a desklet at the top of the screen on the primary monitor.

Enable the monitor above the primary monitor.

The desklets will have moved location, relative to their placement on the monitor they were added to.

Expected behaviour

That desklets will remain fixed in place on a given display, regardless of the configuration of other monitors on the same system.

Other information
In the attached pictures, which include the entire desktop of 2/3 monitors, you can see in the desktop with just two monitors that the desklets for disk space (the green circles on the left), appear about half-way down the desktop, and other desklets appear in the top-right.

In the picture with three monitors, the diskspace desklets appear at the top of the screen (desired location), the other desklets that had been in the top-right, are now gone (presumably moved up into the invisible area not covered by any monitor), and two other desklets (pictures and binary clock) are now visible on the bottom-right (again, presumed to be in dead space below the primary display when just two monitors are on).

This is an issue on several systems for me:
On this system (a desktop), I turn this top display on and off regularly (it is a 40" HD that is connected to many things)

. If it's on, there's a fair chance that items may launch to that display, even though I may have something else displaying on that monitor (as in, it's not displaying this machine).

On my laptops, they are sometimes connected to external displays.