Improve Free Memory Feature

[uroybd]

The current settings to free memory after a certain time of inactivity is a very good addition. However, 2 more options will be much appreciated:

1. A settings option to set the Pinned tabs always active.
2. A per-tab setting to mark a tab as always active.

[metastable-void]

Pinned tabs should already be marked as active. Per-tab settings need changing internal models for tabs/sessions, so we may consider that feature for version 13.0.0.