Permissions Plugin: Disabling UI buttons when the plugin is not installed

[sizious]

When the Permissions Plugin tables aren't installed, the UI isn't working properly. Indeed, when the users_groups_permissions table is missing, all the actions made in the screen aren't taken into account. This is really confusing for the end-user.

Now, the User Groups Permissions is disabled in the Plugins page:

In the Groups page, all the buttons are disabled and a message is shown: