Is your feature request related to a problem? Please describe.
When PCManFM-Qt is not running as a daemon there will not be a dbus interface for applications to open directories using PCManFM-Qt.

Describe the solution you'd like
Create a .service file in /usr/share/dbus-1/services/ so if any applications sent a request to the dbus interface it will automatically start the PCManFM-Qt daemon to handle the request. With the .service file , dbus knows what binary it should execute if the interface is not avialable.

For example the Thunar file manager has /usr/share/dbus-1/services/org.xfce.Thunar.FileManager1.service:

[D-BUS Service]
Name=org.freedesktop.FileManager1
Exec=/usr/bin/Thunar --gapplication-service
SystemdService=thunar.service

For PCManFM-Qt it should be like in /usr/share/dbus-1/services/org.lxqt_project.PCManFM_Qt.FileManager1.service:

[D-BUS Service]
Name=org.freedesktop.FileManager1
Exec=/usr/bin/pcmanfm-qt --daemon-mode

I don't know if we should provide a /usr/lib/systemd/user/thunar.service, like done in Thunar. In Thunar they have:

1. /usr/lib/systemd/user/thunar.service
2. /usr/share/dbus-1/services/org.xfce.FileManager.service
3. /usr/share/dbus-1/services/org.xfce.Thunar.FileManager1.service
4. /usr/share/dbus-1/services/org.xfce.Thunar.service

Read More: https://dbus.freedesktop.org/doc/dbus-daemon.1.html

Related issues: #1194, helloSystem/ISO#71