Issues with navigation link activations to other base Cacti pages

[TheWitness]

Describe the bug
If you wish to link to say the 'graphs' page from a third party page, the Cacti menu api will loose the selection of the page that was navigated from. Instead Cacti should, if it does not find the link in the current navigation, maintain the prior menu selection.

To Reproduce
Steps to reproduce the behavior:

1. Create a third party plugin that maintains it's own menu
2. Have a page on that menu that let's you drill down to the graph_view.php?action=preview, or some other base package without leaving the current navigation
3. Notice that the selected menu item is de-selected leading to confusion on the page.

Expected behavior
Cacti should keep the prior navigation in place if there is no matching navigation on the in the current menu.

[TheWitness]

Commit that fixes is here: 512b704#diff-e65e4069723fadf5732290878dd58a2c