Improves form handling for checkboxes.

Checkboxes now generate an array. This allows multiple checkboxes with the same
name and differing values to generate a nice list of values, allowing the
python side to do:

```
  'value' in e.data['name']
```

It also allows a nice check against a single checkbox, since the empty list is
seen as False:

```
  # if unchecked:
  if not e.data['name']:
      pass
```
Improves form handling by using correct tag.

The target configuration is now grabbed from the form tag's action attribute if
it is not explicitly given.