Migrate Umineko specific builds to a fork

[drojf]

Currently, (due to me), the releases page contains twice as many executables as normally needed, and the executables are named "umineko".

In addition, the Windows executables have an Umineko icon, which is not useful for people who just want to use the engine in an arbitrary game,

This is due to some changes to the build system I made some time ago (I guess I thought we would only use this internally for some reason), plus Olga did the icons for us.

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I would fix this by forking this repository to ponscripter-fork-umineko, and hosting the Umineko specific stuff there. Also, adding a readme to explain that this repo for internal use only.

Then on this repository, reverting any changes I made to the build system, and reverting the icon change.

[drojf]

I've implemented this in 97e597d and 4ef9aa3

umineko executables will now be hosted at https://github.com/07th-mod/ponscripter-umineko (I haven't set this up properly yet).

Also, the installer no longer downloads from this repo, it downloads from https://github.com/07th-mod/ponscripter-umineko, so we can release executables in any format/naming scheme without breaking things.