After remembering an editor selection, for opening a large file,
canceling the respective dialog results in opening the previously
selected editor.

This change resets the values stored in DialogPromptForEditor, prior to
opening the editor selection dialog. This ensures canceling or closing
the dialog yields correct results.

Change-Id: I84937148c930b26cd5abb9c155507b46a009bb36
Signed-off-by: Simeon Andreev <simeon.danailov.andreev@gmail.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/191609
Tested-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-by: Andrey Loskutov <loskutov@gmx.de>