[IMP] account: Bank statement reconciliation

It was impossible to make a partial reconciliation with several lines.
Each time a new line is proposed for the reconciliation, the previous
partial reconcialtion is canceled. The partial reconciliation can just
be proposed on the last selected line when:
self.get("balance") * (last_line.debit - last_line.credit)<0

addons/account/static/src/js/account_widgets.js

@@ -1303,8 +1303,14 @@ openerp.account = function (instance) {
 
             // Show or hide partial reconciliation
             if (self.get("mv_lines_selected").length > 0) {
-                var propose_partial = self.getCreatedLines().length === 0 && self.get("mv_lines_selected").length === 1 && balance_type === "greater" && ! self.get("mv_lines_selected")[0].partial_reconcile;
-                self.get("mv_lines_selected")[0].propose_partial_reconcile = propose_partial;
+                var last_line = _.last(self.get("mv_lines_selected"));
+                var propose_partial = self.getCreatedLines().length === 0
+                    && balance_type === "greater"
+                    && Math.abs(self.get("balance")) < Math.abs(last_line.debit - last_line.credit)
+                    && self.get("balance") * (last_line.debit - last_line.credit) < 0
+                    && ! last_line.partial_reconcile
+                    && ! last_line.already_paid;
+                last_line.propose_partial_reconcile = propose_partial;
                 self.updateAccountingViewMatchedLines();
             }