[register] Checkbox any value other than 0x20 (ASCII space) is true.

Because it can be X or U+2716 now and could be something else
in future.

Fixes in particular the disappearing tax-included and tax-table
in the Invoice/Bill entry register.

gnucash/register/ledger-core/gncEntryLedgerModel.c

@@ -348,7 +348,7 @@ gnc_entry_ledger_get_taxable_value (VirtualLocation virt_loc,
         const char *valstr =
             get_taxable_entry (virt_loc, translate, conditionally_changed,
                                user_data);
-        if (valstr && *valstr == 'X')
+        if (valstr && *valstr != ' ')
             return TRUE;
     }
     return FALSE;

gnucash/register/register-core/checkboxcell.c

@@ -51,7 +51,7 @@ gnc_checkbox_cell_set_value (BasicCell *_cell, const char *value)
     CheckboxCell *cell = (CheckboxCell *) _cell;
     gboolean flag = FALSE;
 
-    if (value && *value == 'X')
+    if (value && *value != ' ')
         flag = TRUE;
 
     gnc_checkbox_cell_set_flag (cell, flag);