Fix inventory selector UI does not display non-selectable items in some situations

[olanti-p]

Summary

SUMMARY: Bugfixes "Fix item selection menu for various actions would sometimes silently omit non-electable items rather than print the reason"

Purpose of change

Fixes #41010 which was a source of confusion in #41703, #40376 and #41926

When none of the entries in a column are selectable, inventory_column::allows_selecting() returns false, causing inventory_column::get_entries(...) to return an empty vector instead of all those non-selectable entries.
That caused inventory_selector::toggle_categorize_contained() to lose these entries.

Describe the solution

It looks like inventory_column::allows_selecting() was intended as a per-column setting to differentiate real and decorative columns, while inventory_column::activatable() checks whether there are any selectable entries in the column. So, the solution is to change the 1st to return true rather than falling back to calling the 2nd.

Testing

The multidrop menu (the one that requires the whole allows_selecting thing) also seems to be working