shell: window: properly deactivate document actions, and keep track of stateful action toggle-find

To be used with gtk_widget_action_set_enabled an action has to be part of the window's action group, and cannot be installed differently.

Thus, we need to store the document action group separately, and activate from that group directly.

Fixes: 4e8b3974 ("shell: reorganize actions for PpsWindow") Closes: #138 (closed)

The toggle find state is used for the search toggle button, and the toggle find state ensure that the view also gets the search results highlighting.

Fixes: b057a91d ("window: toggle search on Ctrl+F with no text selection") Closes: #139 (closed)