Input filters aren't sorting correctly in FormsAPI

Form elements with the same weight are sorted randomly each page load. This primarily effects core in regards to filter formats - our node types have a Body of 0, and our filter formats do as well. This causes, randomly, the filter format to appear ABOVE the Body. There's also the danger of multiple textareas (each with their own filter format), all with the same weight: the chaotic sorting creates a situation where you have no idea which input filter applies to which textarea.

There were lots of possible ways to solve this, including a new form element type (textarea_filtered), a new element attribute (#filter => 1), unique weights (0.4535), alphabetical sorting with names like "body" and "body_filter", and so forth. Ultimately, chx and I decided that using FAPI containers, or "branches", would be the simplest solution, and also utilizes what FAPI already provides. Branches are containers, just like fieldsets (only non-visual), and the items in the branch sort according in their own weight table. Thus, to ensure that the textarea and the filter always appear that way visually, without conflicting with any other same weight elements, we just give all our filterable textareas in core their own branch, which includes ONLY the Body and the Input Filter.

The patch also cleans up some double newlines.