#1 .txt extension uneccessary in .grid save dialog

[Mackenzie Cowell]

When saving a grid during "addressing/gridding" by
using the "save current grid" menu item under the
"File" menu, the save-file dialog box that appears
automatically appends a ".txt" extension to the
suggested file name. This is visible to the user, but
hard to catch (in my experience). Although the result
- a grid file named like this: "expfilename.txt.grid" -
is totally benign, it is a little confusing and exists
simply because no one has taken the time to fix it.
Speaking of which, the fix should be really simple -
just tracking down the string that is suggested to the
save-file dialog.