Revert "Always use pre-GL3 `glGetString(GL_EXTENSIONS)`."

We need better solution for that.

This reverts commit c4090e8f2fae84e2f3776c91b80e3e395e11bd64.
Load all extensions no matter which ones are supported.

It seems easier to try to load a few more function pointers than going
through a few nested loops and for each extension returned from
GL_EXTENSIONS (hunderds of them) test whether that one is one of our
EIGHT! required extensions.

Anyway, this overengineered and overcomplicated codebase drives me nuts.
I want something simple and not this huge codebase which is trying to
look like it is able to create loaders in any imaginable language and
coding style ever invented while in fact it can't handle simple
requirements.
Added .kateconfig.

Tabs, tabs everywhere.