Hi!
First, to clarify: SFML is not an engine.
It's simply a library that is used to help with certain multimedia (and some other) tasks within a programming language.
Programs are written in C++ (or some binding) and SFML is called upon for some tasks. However, management of resources are, like any other program you may write in the same language, managed by your program in that language.
A simple solution is to collect them all into a struct/class, maybe using vectors to collect similar types. That way the class object can be passed around as a single object.
There are, of course, better and more complex solutions. One example of a resource manager with a better design is included in the
Thor library.
To create an IDE based on SFML, you are likely to need some form of GUI. Although it's possible to create your own using SFML, there are already some available.
e.g.
SFGUI,
TGUI,
ImGui.
These are GUI solutions using SFML itself. However, you may wish to use more native GUI systems. Commonly mentioned ones are: Qt and wxWidgets.