Changed from ctrl to alt because otherwise it's easy to accidentally hit ctrl up…

Changed from ctrl to alt because otherwise it's easy to accidentally hit ctrl up and jump when you don't meant to while pruning an image list.

tools/imglab/src/metadata_editor.cpp

@@ -336,7 +336,7 @@ on_keydown (
             last_keyboard_jump_pos_update = 0;
         }
 
-        if (key == 'd' && (state&base_window::KBD_MOD_CONTROL))
+        if (key == 'd' && (state&base_window::KBD_MOD_ALT))
         {
             remove_selected_images();
         }
@@ -578,7 +578,7 @@ display_about(
                         "a rectangle selects it and the delete key removes it.  You can also mark "
                         "a rectangle as ignored by hitting the i key when it is selected.  Ignored "
                         "rectangles are visually displayed with an X through them.  You can remove an image "
-                        "entirely by selecting it in the list on the left and pressing ctrl+d."
+                        "entirely by selecting it in the list on the left and pressing alt+d."
                         ,0,0) << endl << endl;
 
     sout << wrap_string("It is also possible to label object parts by selecting a rectangle and "