gnome-panel should recognize installed applets

[jtojnar]

Panel applets are being picked up from GNOME_PANEL_APPLETS_DIR environment variable in one place and a completely hardcoded path in another.

First, we will need to patch the latter manager to use an environment variable as well.
Second, we will need to do something similar to what MATE is doing. Or better, create gnome-panel-with-plugins derivation.

Originally posted by @jtojnar in #47451 (comment)

[stale]

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[chpatrick]

@jtojnar I'd like to tackle this, do you have any pointers? We need to get it to pick up gnome-applets at least right?