`move position` is treated globally rather than relative to the current output

## I'm submitting a…

[x] Bug

[ ] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior
In a dual-monitor setting, whenever I launch a floating window which has its position set, it opens in my external monitor.

## Expected Behavior
I expect the floating window to open in the current workspace.

## Reproduction Instructions
I have 2 situations in which the position of a floating window is set and the problem occurs in both of them (while I have no problem with floating windows which are free to open in the default center position).

1st situation: floating window position set by i3:

```
for_window [title="qalc"] floating enable, move position 1125 878, resize set 800 200
```
If I don't set the position (`for_window [title="qalc"] floating enable, resize set 800 200`), the "qalc" window opens in the correct workspace.

2nd situation: floating window position set through yad (`--geometry` flag):
(I created this clumsy and ugly hack to get info on my backlight level when I adjust it)

```
bindsym XF86MonBrightnessUp exec --no-startup-id "xbacklight -inc 10 && xbacklight -get | xargs printf \\"%.*f\n\\" 0 | yad --text-info --sticky --no-buttons --undecorated --timeout 1 --geometry 10x10-10+3"
```

Same behaviour in both cases: when I am in a workspace on the external monitor, things work as expected. But when I am in a workspace on my primary monitor (laptop screen), the floating window still opens in the visible workspace of the external monitor (and the focus is thus now on that workspace).

## Environment

Output of `i3 --moreversion 2>&-`:

i3 version: 4.17.1O

- Linux Distribution & Version: Arch Linux (5.4.10-arch1-1)

- Are you using a compositor (e.g., xcompmgr or compton): no