Don't install SFML when installing the project

[JonnyPtn]

Just a case of adding the subdirectory with EXCLUDE_FROM_ALL. In cmake 3.28 you can pass this property directly to fetchcontent which is nicer but presume we're a way off using that version As discussed, we are happy to move to 3.28 as part of this

Also had to change the fetch content name to lowercase, which is required for the source/binary dir variables to work correctly and is good practice anyway.

closes #6

edit: bonus side-effect of users not having to build sfml modules they aren't using

[eXpl0it3r]

I believe it makes sense, as it otherwise makes the install target kind of unusable

[JonnyPtn]

didn't realise the decision had been made not to support installation in the template a few months ago (before changes were requested here) which makes this kind of redundant (edit: suppose not entirely redundant as it makes it generally more efficient)

Seems silly imo to not support it, it's one of the most common questions new users ask about and would be useful for many people with minimal effort

[ChrisThrasher]

Raising the CMake requirement beyond CMake 3.22 means that Ubuntu 22 users no longer have a seamless experience using this template. There are a number of ways to remedy this in decreasing order of what I would personally recommend.

1. Upgrade to Ubuntu 24 LTS. This ships with CMake 3.28.
2. Add Kitware's Apt repository which will give you the latest release of CMake.
3. Tweak the template's CMakeLists.txt to remove the use of EXCLUDE_FROM_ALL and SYSTEM.
4. Use an older commit of the template before this PR was merged.