Fixed positioning of the image/layer Drag&Dropped to the Krita Canvas

Now the Drag&Drop of the image or a layer to the canvas works
in the following way:

1) [no modifiers pressed]

* Dropping Krita Layer (from Layer Box):
Positions to the original offset in the original image. If it doesn't
fit into new image, it drops to the center of the new image.

*Dropping an *image* (not URL) from Chromium or other application:
Drops to the center of the image.

2) [Shift modifier is pressed]
Both types of pastes are recentered near the center of the new image.

3) URL's are always dropped as usual to the top-left corner of the image.