Avoid exception, if there are no changes

This happened in commit 486a3c24b6b92f1345c280f1e17e66c54e86cd92

```
TypeError: rec.changes is undefined                              w2ui.js (Zeile 3538)
...revious: (rec.changes.hasOwnProperty(col.field) ? rec.changes[col.field]: old_va...
```
Avoid exception, when extending selection into summary row

```
TypeError: obj.records[i] is undefined
newSel.push(obj.records[i].recid);
```
Fix grid context menu position for Firefox

Firefox does not have offsetX. pageX is absolute and lets the menu jump all over the place.

Solution based on http://stackoverflow.com/a/21780843/2127439
Merge pull request #478 from wolfmanx/fix-editchange-exception

Fix grid exceptions
Merge pull request #479 from wolfmanx/fix-grid-menu-position-ff

Fix grid context menu position for Firefox