add --select compatibility flag

[ItsZariep]

Some scripts rely on the --select flag to highlight files in file managers. While caja, dolphin, and nautilus support this flag, pcmanfm-qt currently does not.

Currently, pcmanfm-qt triggers an argument parser error when using --select, and if not, it open the file with the user selected mime app.

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This simple PR adds support for the --select flag in pcmanfm-qt using the already existing Application::ShowItems method.

With this change, --select will be accepted by pcmanfm-qt without errors, improving compatibility with scripts expecting this flag.

[tsujan]

I'm afraid this is a duplication of a standard D-Bus method, which is independent of FM:

qdbus6 org.freedesktop.FileManager1 /org/freedesktop/FileManager1 ShowItems 'FILE_PATH' "0"

While caja, dolphin, and nautilus support this flag…

While they support it, it isn't a standard, and we avoid duplication.

BTW, Caja and some other GTK file managers were forked from Nautilus.

[tsujan]

… improving compatibility with scripts expecting this flag.

Scripts should be written in such a way that's independent of FM. Well-known FMs follow freedesktop.org standards.