doc: minor formatting changes.
Integrated plugins into build system and updated the documentation.
Plugins: skip the unimplemented test case instead of failing whole test.

It is better to run automatically all tests than to manually check that
only this one error is present.
Plugins: remove the need for FreeTypeFont in MagnumFontConverter test.
Build all the plugins in development PKGBUILDs.
Plugins: fixed ES build of TgaImageConverter.
Fixed ES build without deprecated API.
Plugins: mark the includes as inter-project, not external.
Math: doc++
SceneGraph: doc++
Platform: deinline default event handlers.

They can't be used as inline anyway.
Platform: added no-op GlutApplication::keyReleaseEvent().

GLUT doesn't have key release events, this function is included only for
compatibility with other toolkits and doesn't get called at all.
Platform: document the intention better.
Platform: added ScreenedApplication class.

Better API for handling more than one application screens (context
switching, event propagation etc.). Taken from Push The Box, updated to
current coding style and templated.
Shaders: minor cleanup.
Shaders: fixed Flat shader.

It is now [0] days since last copypaste error.
doc: documented Plugins directory.
Shaders: okay, I admit, my NVidia really _is_ forgiving in GLSL ES.

The tests didn't catch this at all.
Handle WITH_* options for utilities better.

I.e., don't offer them on platforms which don't support them.
TextureTools: renamed magnum-distancefield to magnum-distancefieldconverter.