bindings: allow using <Backspace> to scroll back up in the help viewer

Backspace is a more logical counterpart of Space than minus.

src/global.c

@@ -1082,6 +1082,7 @@ void shortcut_init(void)
     add_to_sclist(MMAIN|MHELP|MBROWSER|MLINTER, "^Y", 0, do_page_up, 0);
     add_to_sclist(MMAIN|MHELP|MBROWSER|MLINTER, "F7", 0, do_page_up, 0);
     add_to_sclist(MMAIN|MHELP|MBROWSER|MLINTER, "PgUp", KEY_PPAGE, do_page_up, 0);
+    add_to_sclist(MHELP|MBROWSER, "Bsp", KEY_BACKSPACE, do_page_up, 0);
     add_to_sclist(MMAIN|MHELP|MBROWSER|MLINTER, "^V", 0, do_page_down, 0);
     add_to_sclist(MMAIN|MHELP|MBROWSER|MLINTER, "F8", 0, do_page_down, 0);
     add_to_sclist(MMAIN|MHELP|MBROWSER|MLINTER, "PgDn", KEY_NPAGE, do_page_down, 0);