Change the object position form field UI from number input to sliders

[pablodiegoss]

When uploading an object the user must inform the base position vertically and horizontally, the default being 0 for both. as so:

However this is non-intuitive and most users have no idea what those numbers really mean. This issue is to substitute both fields for range sliders

a range slider for each field which control the position of a visible square that represents a marker and an object

This will better indicate for the user that the horizontal and vertical position alter where the object will appear in relation to the marker.

Note: Example images taken from here, with a tutorial that explains them

[vjpixel]

This feature is breaking the Object's edit page. See #429

@pablodiegoss