Fixes #51316 (actually, it fixes the _real_ problem).

* src/dialogs/dialog-plugin-manager.c
(cb_pm_button_activate_plugin_clicked): If the plugin is already
active, remove it from "marked for deactivation" list.
(cb_pm_button_deactivate_plugin_clicked): If the plugin cannot be
deactivated, optionally add it to "marked for deactivation" list
(if the user wants to).
(cb_pm_clist_row_selected):
Make "Activate plugin" button sensitive for some active plugins (for
those on "marked for deactivation" list).
(cb_pm_clist_row_unselected):
Bugfix: don't free pm_gui->current_plugin_id string.
(update_plugin_manager_view):
Display the plugin state after its name ("in memory", "marked for
deactivation").
(dialog_plugin_manager): Plug leak.

* src/plugin.c
(plugin_info_force_mark_inactive,
plugin_db_mark_plugin_for_deactivation,
plugin_db_is_plugin_marked_for_deactivation): New functions.
(plugins_shutdown): Set all plugins on "marked for deactivation"
list as inactive before saving active plugins in configuration.

* src/plugin.h:
(plugin_db_mark_plugin_for_deactivation,
plugin_db_is_plugin_marked_for_deactivation): New functions.