Plugin menu do not show when run thunar --daemon

I can not compile thunar master at the moment, so I backport below commit to xfce-4.16 branch to test, maybe this issue does not exist in master.

8a3b98da Look for thunar plugins at $THUNARX_DIRS (Issue #748)

1. cd /path/to/thunar
2. RUN: THUNARX_DIRS=/usr/lib/x86_64-linux-gnu/thunarx-3:/usr/lib/x86_64-linux-gnu/thunarx-3 ./thunar/thunar --quit
3. RUN: THUNARX_DIRS=/usr/lib/x86_64-linux-gnu/thunarx-3:/usr/lib/x86_64-linux-gnu/thunarx-3 ./thunar/thunar --daemon

Open another xfce terminal:

1. THUNARX_DIRS=/usr/lib/x86_64-linux-gnu/thunarx-3:/usr/lib/x86_64-linux-gnu/thunarx-3 ./thunar/thunar

2. Close all thunar windows. 3 Run again: THUNARX_DIRS=/usr/lib/x86_64-linux-gnu/thunarx-3:/usr/lib/x86_64-linux-gnu/thunarx-3 ./thunar/thunar