Prevent changing analytic account of auto generated tax line and revert ab797f0cd8f76b

Analytic account should be edited only if the line is manually typed. This is related to issue #374 and https://bugs.launchpad.net/ocb-addons/+bug/1084822

Also, revert ab797f0cd8f76be232fcd918eafa52b847102504 "[FIX] account: Error when manually adding analytic account in the generated tax lines on an invoice" as the target issue was resolved by commit 18b7b28bc1ee676a8d245f50821d4a9aeb844158 except when the user manually changes the analytic account which should not be allowed and is fixed by this PR.

The reverted patch introduces a new problem; if the invoice has multiple lines with different analytic accounts and similar tax, they will generate one tax line with the analytic account of the first invoice line.