Browsing templates was easier when popup displayed all template host groups, not only sub-groups.

Steps to reproduce:

1. Navigate to Configuration -> Actions -> Autoregistration actions
2. Press "Create action" and switch to "Operations" tab.
3. Click on "Add" button and choose "Link to template" operation type.
4. Open multiselect popup. 

Result:
You cannot browse all templates in "Templates" host group. Only sub-groups are available in Host group selector.

Expected:
Before Zabbix 5.0 there was also "Templates" host group available so user was able to see all available host groups.