#423 add "use current position" option to object creation menu

[Jason]

Add the capability to use the current position when creating/modifying
an object.

Add a button "current position" to the "Location" area of the create and
modify popups. This should be the GPS position; if no GPS fix is
available, not sure if simply using the current position is appropriate,
with or without some sort of warning? The button would copy the
current position into the coordinate boxes.

On the object creation popup, also add a checkbox to indicate that the
current position is always to be used.

-Jason