screen: Make some strings shorter & add subtitle for the "Automatic Screen Lock" row

The strings related to screen blank are long and cumbersome.

This MR makes these strings shorter, following the advice from: #1433 (comment 1607990)

Also, this MR adds a subtitle for the "Automatic Screen Lock" row.

Before	After

Since the issue #1433 (closed) was already fixed with the GTK 4 port, I didn't add the issue link to the commit message.

/cc @aday