Ensure that input focus is returned to correct window when i3lock quits

[CometStorm991]

Description

• The input focus does not always return to the previously focused window after i3lock-color quits.
• For example, when the --composite option is enabled, the parent window set by parent_win = composite_reply->overlay_win; in xcb.c can be different from the window from which focus was stolen.
• If i3-lock fails to grab the pointer and keyboard on its first attempt, the input focus will be set to i3lock and will later revert to the parent window (which may not be the correct window when the --composite option is enabled) as shown below (comments removed)

    auth_state = STATE_AUTH_LOCK;
    if (!grab_pointer_and_keyboard(conn, screen, cursor, 1000)) {
        DEBUG("stole focus from X11 window 0x%08x\n", stolen_focus);

        xcb_set_input_focus(conn, XCB_INPUT_FOCUS_PARENT /* revert_to */, win, XCB_CURRENT_TIME);
        ...
    }

• Incorrectly reverting the input focus can cause user input in DE/WMs to seem "frozen" since keyboard input may not appear to work (although the mouse may work).
• Adding xcb_set_input_focus(conn, XCB_INPUT_FOCUS_PARENT /* revert_to */, stolen_focus, XCB_CURRENT_TIME); to the end of main(...), ensures that focus returns to the correct window stolen_focus.