What problem are you trying to solve with this idea and patch?  A popup go to line box is standard enough for editors that beginners have no problem with it.

That asked, I read the sublime-text doc on the go-anywhere bar.  While it is a cute idea, I think that trying to mix multiple functions like this is mis-conceived.  Mixing control signals (the prefixes) with content is *always* a problem.  Suppose one wanted to search for the string ': 10'?  The problems people have with \ in string literals and various control chars in regexes also illustrate the point.  

Once a user hits alt-g (not cntl-g, which IDLE uses from find-again), IDLE knows to expect a string of digits that can be fed to int().  Replacing the clear 'go to line number:' prompt with the anti-mnemonic ':', mixed in with the input, is to me a regression.

Here is an alternate idea for streamlining line number entry.  In the status bar, replace the labels that display line and column with entry boxes.  Alt-G sends the cursor to the line# entry. The color is changed to indicate entry mode.  Escape undoes any change and sends the cursor back where it was.  Click send the cursor to the click spot.  Return goes to the (new) line and send the cursor there.  Tab moves the cursor to the column box for possible column entry.  A click on either box switches it to entry mode.

While goto column is much less needed, I can think of use cases and notice that the goto box for Notepad2 on Win10 includes a separate column entry box.  

I think someone else proposed adding a simple find box with default options to the status bar.  This would *not* replace the current box.  For simple searches, it would solve the problem of the box initially obscuring text, now that it no longer disappears when Next is clicked.