Improving the "missing DMS Pro Tools plugin" notification

The pl_has_dms_plugin()function triggers an irrevelant/inaccurate alert: the Pro user is told to install the plugin, while it can be already installed but not activated. Notification text changed accordingly.
Merge branch '1.1' of https://github.com/pagelines/DMS into 1.1
Merge pull request #579 from erwanlescop/1.1

Improving the "missing DMS Pro Tools plugin" notification