This also happened before applying
    Manage what happens when a PanelViewerBase (e.g. using Shift+F3) tries to open a nonexistent local file
    https://phabricator.kde.org/D27258
or
    Manage the use of Shift+F4 when opening a local file that exists, or a remote folder or remote file that exists
    https://phabricator.kde.org/D27259
though some of the code that was introduced in https://phabricator.kde.org/D27259 was reused in the present code review.

Using F4 to try to open a:
    - local file that exists: /etc/passwd
    - local folder that exists: /etc
    - remote folder: smb://user@IP_ADDRESS/PATH/EXISTENT_FOLDER
    - remote folder: fish://user@IP_ADDRESS:PORT/PATH/EXISTENT_FOLDER

Using Shift+F4 to try to open a:
    - local file that exists: /etc/passwd
    - local folder that exists: /etc
    - remote folder: smb://user@IP_ADDRESS/PATH/EXISTENT_FOLDER
    - remote folder: fish://user@IP_ADDRESS:PORT/PATH/EXISTENT_FOLDER

Performing the tests using the default internal editor and also /usr/bin/kate.

Performing the tests using e.g. Kubuntu 18.04 LTS and Kubuntu 19.10.