Like the topic say I want to overwrite/change the "Menu Manager: Edit Menu item" layout. To illustrate my question:
In the picture whiche is shown I want to change the labels: Layout, Option, Integration.... and add some other options to it. How can I do it? Or is this even possible?
In order to change the text, simply use language overrides, google is your friend.
In order to add functionality, let's first of all explain what we're talking about to ensure we're on the same page.
Joomla components have views which can have one or more layouts, i.e.
/components/com_content/views/category/tmpl/ contains two layouts, blog and default.
A layout can additionally contain an .xml manifest (in our case, blog.xml and default.xml) allowing us to create a menu item for the specific view/layout combination. The .xml file contains the parameters that the user will set, you can add your own as well.
When you want to change Joomla, usually there is a way to do so without touching the core, which would be pretty bad, as any Joomla! updates would break your work.
For the view layouts a special feature called template override was developed, which allows you to create an alternative to the view layout in a safe place (under your template folder, in this case your admin template), and this is the most elegant and effective way to achieve your result.
Beware though, you are just creating a layout, most likely you will want to add functionality, if it's complex you might be better off creating a dedicated component to keep the code clean. Or you can just put all the logic in your view, query the database from there. But in this latter case, get paid, and run away. Never answer the phone to the customer again.
A final alternative is to write a system plugin that will manipulate the page markup after it was generated in the event onAfterRender(). This is a simple and good approach if you only want to add a button or make minor changes, but if you do anything more than that, see the above advise about running away.
