Remove duplication of find_tags enumeration
Enable "Open Files" search option in Find window

This seems to have been overlooked since most of logic was already implemented.

There is one known issue that exists when restoring sessions. Namely, after restoring a session only those documents (tabs), which have been actively "viewed" are considered "opened".
Prefer `NULL` to `nop:` in most cases

This change was prompted by the appearance of the `tabSizePopUp` button in the status bar. After dismissing the menu, the title would be disabled.

To ensure we do not have similar issues elsewhere, let's prefer the use of `NULL` to `nop:` when assigning the action for menu items unless we are sure that the menu item should always be disabled (e.g., the item is used as a label).
Indent menu items in `tabSizeMenu`

This allow the check mark to be properly spaced with respect to its item.
Use path::escape when creating TM_SELECTED_FILES

In addition to being redundant, now that we have a global escape function, the custom shell escaping code had a bug when the path contained more than one apostrophe.