Document Properties dialogue moves offscreen after repeated use
#### Steps to reproduce:
- open Inkscape (no file needed)
- click File
- click Document Properties (note dialog position)
- close dialog
- click File
- click Document Properties (notice new position)
- repeat operation and dialog moves off the screen
#### What happened?
Document Properties dialog moves toward the right hand side of the screen every time the dialog is opened.
#### What should have happened?
Dialog should have opened in the middle of the screen or the same position it was in previously.
#### Inkscape Version and Operating System:
- Inkscape Version: Inkscape 0.92.4 (5da689c313, 2019-01-14)
- Operating System: Windows 10 64-bit
- Operating System version: Build 18362.239
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