Hide notes buffer whe no notes available

[keram]

This revisits #148 and improves the user experience when Idris does not report any errors.

Previously in such case the user is left with empty buffer in some of the windows. After this change if there is nothing to display the window the notes either display previous buffer or it is closed completely if notes buffer was the only buffer displayed in it.

Example:

• single window

• two existing windows

[keram]

Considering renaming this function as it handles full life cycle of the notes not just showing them.

[keram]

This message is "never" shown as it is quickly superseded by 2 another messages.
Messages in messages buffer.

...
Preparing compiler note tree...
Press q to close, return or mouse on error to navigate to source
Wrote /idris/idris2/src/Idris/Parser.idr
Evaluation returned an error: Error(s) building file src/Idris/Parser.idr.

Will remove it as part of separate MR

[keram]

I'm also not convinced about this code

        ;; Clear the contents of the compiler notes buffer, if it exists
        (when (get-buffer idris-notes-buffer-name)
          (with-current-buffer idris-notes-buffer-name
            (let ((inhibit-read-only t)) (erase-buffer))))

in idris-load-file which was solution to #123 as it adds extra responsibilities to this function and there may be better ways to notify user about changes in the notes buffer.
(mode line change, fringe colour change etc.)