bug#27435: patch for dired-mouse-find-file-other-window

[Tak Kunihiro]

>> On dired I use `RET', `o', and `C-o' on different occasions and I
>> prefer mouse click to work as `RET' instead of `C-o'.  This is
>> especially true when there are more than three windows in a frame.
>> 
>> The patch lets mouse click on dired visit a file or directory in
>> <this-window> not in <other-window> with following customization.
>> 
>>   (setq dired-mouse-find-file-func 'find-file)
> 
> But you could have the same effect by rebinding mouse-2 to another
> function.  We just need to add such a function, which should work like
> dired-mouse-find-file-other-window, but visit the file in the same
> window.  That sounds like a better alternative for this use case,
> because customizing a function-valued variable is trickier for users
> than rebinding keys.

I agree.

Then the new function `dired-mouse-find-file-this-window' will look
very similar to `dired-mouse-find-file-other-window' but two lines.
Is that OK?