Moved FpsCounterExample class to viewer example.

Was not used anywhere else.
Triangle example can now be run also with OpenGL ES.
Cleaned up CMakeLists.

Renamed executables to all-lowercase. Now every example is
self-contained (i.e. can be built on its own).
New primitives example.

In complexity it is somewhere between triangle and textured-triangle. No
time to write the documentation yet, will be added later.
Updated FindCorrade.cmake and FindMagnum.cmake modules.
Adapted to Magnum changes.
Updated toolchains submodule.