Window-Snap lacks the preview when being invoked by mouse-drag

We recently introduced Window-Snap, but it lacks the preview functionality and therefore what currently happens, might surprise the user.

Reproduction steps

• Open an explorer file browser and make sure it is in non-maximized state
• Drag that window to the left side of the screen, but do not release the mouse button yet

Expected behavior
Win7 draws the rectangle that the window will cover, due to the snapping, with a different semi-transparent colour
Win7_Preview_ok.webm

Observed result
ros does not show the preview, but will instead just snap (later after mouse release), which may surprise the user
0.4.12-RC-52-g3fa352b_lacks_The_Preview.webm

Ftr

• ros logs nothing, therefore I skipped the logs
• The preview is only applied in Windows 7 when you use the mouse to drag. It shall not happen when the keyboard-shortcuts are used.
• in case it would be too difficult to draw the preview rect with a different and semi-transparent colour, I would also accept the ticket as being resolved, if our preview would instead surround the preview-rect with a dotted XOR-line