Sitefinity add new function to page text editor

Question

I can find a lot of documentation on how to add things to the text editor itself, including a place to add additional buttons/functions into the XML through the admin settings.

What I cannot find is where I would then create the new functions that those settings reference. I tried searching my local solution for a few random ones and nothing came up.

For clarification, I'm looking at adding in a button on the editor for pages - or really anywhere in the back end that uses the text editor, but at least in pages. I'm not trying to add a completely new editor in my own widget or something, which would make this really easy as it appears those can simply be declared inline.

Update

Going to Administration -> Settings -> Text editor and clicking "edit" for "Default tool set" gives me a settings XML sheet something like this. I should be able to just add a new tool here in the XML, but I have no idea where the actual code for that tool should live in order to be used by this editor.

<?xml version="1.0" encoding="utf-8" ?>
<root>
  <modules>
    <module name="RadEditorStatistics" dockingZone="Bottom" visible="false" />
    <module name="RadEditorDomInspector" visible="false" />
    <module name="RadEditorNodeInspector" visible="false" />
    <module name="RadEditorHtmlInspector" visible="false" />
  </modules>
  <tools name="MainToolbar">
    <tool name="ToggleAdvancedToolbars"/>
    <tool name="Bold" shortcut="CTRL+B"/>
    <tool name="Italic" shortcut="CTRL+I"/>
    <tool separator="true"/>
    <tool name="InsertOrderedList" />
    <tool name="InsertUnorderedList" />
    <tool separator="true"/>
    <tool name="LinkManager" shortcut="CTRL+K"/>
    <tool name="Unlink" shortcut="CTRL+SHIFT+K"/>
    <tool name="ImageManager" shortcut="CTRL+G"/>
    <tool separator="true"/>
    <tool name="AjaxSpellCheck"/>
    <tool separator="true"/>
    <tool name="PasteFromWord" />
    <tool separator="true"/>
    <tool name="FormatBlock"/>
  </tools>
  //...
</root>