Only update nodes of appropriate types (+ multiple improvements)

Ah well... I know it is always better to split multiple changes in multiple smaller patches. However, I needed this immediately, so here's a patch that includes multiple small improvements:

• In DB Setup queries, only update nodes of appropriate types (instead of updating all nodes). This was previously marked as TODO in the code.
• Implemented proper form submit functions (instead of checking $_POST['op']).
• Removed useless intermediate menu callback functions (now calling drupal_get_form() directly as a callback).
• Renamed the internal functions to be prefixed by _weight to follow common Drupal practice.
• Use db_placeholders() for sanitizing node type names passed to db_query().
• In weight_old_nodes(), use a placeholder in t() instead of prepending the message with the node count.
• Avoid showing the submit buttons in the DB Setup forms when there are no nodes to update anyway.
• The Weight selector is added in the Publishing options fieldset rather than in its own fieldset. Who needs yet another fieldset in that form? Also, in some setups it was appearing in the midst of CCK fields.
• Changed the weight selector's description. The previous description seemed a bit too specific and unlike the descriptions we usually find in core.
• Updated some comments.

Let me know what you think of this. ;-)