Forge form grouping

[HarryR]

The current Forge Form_Group class isn't friendly towards grouping items together, when you add a new group it is appended into the Forge inputs and ends up with a numerical ID.

Previously:

// Construct your $forge object
$mygroup = $forge->group();
$mygroup->input('someitem');
// Results in: $forge->inputs[0]->someitem

The attached patch:

// Construct your $forge object
$mygroup = $forge->group('groupname');
$mygroup->input('someitem');
// Results in: $forge->mygroup->someitem

[PugFish]

Applied in r2470