New plugin detection for Symbian

Symbian can have very long running apps, which expect to pick up new
plugins without a restart. This change makes the plugin factory, which
is used for many internal plugin types, detect plugin changes and
update its list of plugins.

This uses the QNotifyChangeEvent class to watch for plugin directory
changes, including when they do not already exist, including on
removable drives with no media currently present. When a change is
detected, it triggers a rebuild of the plugin library paths, then
rescans for plugins only on the drive that changed.

An alternative implementation could have made use of watching software
installer P&S keys for notification of change. However these are not
triggered by memory card insertion or removal, so file system watchers
are used.

Task-number: QTBUG-20098
Reviewed-by: Shane Kearns

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