XML-Driven floater feature (and documentation)

Changes:
Adds LLUICtrlFactory::getBuiltFloater() to return LLFloater* when a floater has been built.
In llviewermenu.cpp's LLShowFloater::handleEvent():
Return early if the floater_name is empty,
otherwise if no anticipated floater is found, build one,
unless one of the same name is already built, then bring it to focus.



Documentation:
In order to add simple codeless floaters, a general understanding of our XMLs is necessary...
A simple codeless floater can be a few things, perhaps not limited to: media browsers, settings controls, and holders for inventory_panels.

So how do you make them conform to the codeless standard?
For the purpose of these examples, my floater file will always be floater_name.xml
Let's start with a shortened xml menu entry:
<menu_item_call label="Name!" name="name">
  <on_click function="ShowFloater" userdata="floater_name.xml"/>
</menu_item_call>
Alright, that wasn't so hard, just remember the key in this is ShowFloater is the function and userdata is the filename.

The next part is the actual floater, which isn't so different from making a complex floater..
There are two ways to do this, depending upon desired behavior:

The first behavior is to have only one of the floater open at a time, the following short xml exemplifies this:
<floater name="floater_name.xml" title="Name!"/>
Having a floater with this behavior will cause the menu entry to bring it into focus.

The other behavior is to allow more than one of the floater open at a time:
<floater name="name" title="Name!"/>
In this case, the menu entry will spawn a new version of the floater with every click, regardless of previous ones existing.

Granted, these short <floater/>'s probably aren't but examples and will not likely stand alone.
Added simple codeless floaters for My Outfits and Favorites folders, entries are in World menu.
Further notifications.xml touchup.