Math: really install deprecated headers.

Dammit. Sorry about this :-)
Revert "Math: don't allow *::data() to be called on rvalues."

The pointer conversion can be done only explicitly, thus the users will
always know what they are doing. With that change, perfectly valid
things like this couldn't be done (the result of a + b is kept until the
semicolon):

    Vector3 a, b;
    void foo(float* data);
    foo((a + b).data());

Moreover this conversion wasn't even properly tested, leading to issues
as in mosra/corrade@781f5df38a7b1b366f3de477dd5fe641eca6ed20.

This reverts commit add989703e694f7719e015cc12717d9953de0322.
Made ImageReference data pointer constant.

ImageReference is supposed to be wrapper around existing data array of
some type which is passed to Magnum functions. Thus modifying the
original data array (which must be kept somewhere for proper deletion)
is much better option than casting the pointer returned by data() to
some proper type and then operating on that. Hopefully this won't break
any existing code.
Disallow conversion of const {Image,Trade::ImageData}&& to ImageReference.

The conversion is done implicitly, thus this seemingly innocent code
would be allowed:

    ImageReference2D reference = Image2D(...);
    foo(reference.data()); // crash on access, data already deleted
Documentation updates for Image classes.
Added release() function to Image and Trade::ImageData.

Allows the image to release ownership of the data.
Platform: no need to have GlutApplication::redraw() virtual.

The other implementations aren't virtual too.
Platform: documentation updates.

Sdl2Application is now taken as base implementation (it was GLUT
previously) and all others are copying/referencing the documentation
from it. When SDL2 is included in all major distributions (Ubuntu, I'm
looking at you), it will replace GLUT as the default application.
Platform: cleaned up unneeded includes.

Also deinlined pure virtual function implementation.
Platform: unified and cleaned up context creation.

Added missing tryCreateContext() implementations. Error messages are
printed only by tryCreateContext(), createContext() is only a thin
wrapper which exits the application if tryCreateContext() fails and
doesn't print any additional information on the output.

Hopefully I didn't break anything :-)
Platform: no need to have convenience overload of internal constructor.
Platform: add default argument to createContext().

Somebody would just want to defer context creation after parsing
arguments or doing some validations without any particular setup, thus
having to write even the {} is annoying.
Platform: there is no point in disabling copy/move of Configuration.

The class isn't heavy at all and we don't need to maintain only one
instance.
Platform: WindolessGlxApplication::Configuration can be a POD.

No need to create symbols for empty constructor/destructor.
Platform: disable copy/move of Application classes.

They are something like singletons (or they expect that behavior
internally), moreover some code might hold pointer to them, thus
movement is not desired.
Platform: publicly document copy/move inability of Event classes.

We need to create one instance, send it to subclasses and then check its
state, thus we expect that the user always operatres with the original
instance.
Platform: make virtual functions private, rendering protected, other public.

 * The default (empty) implementation of virtuals shouldn't be called,
   thus this effectively protects the user from doing it.
 * Only the application itself knows best when and how to call
   rendering-related functions such as swapBuffers() and redraw(), thus
   they are protected.
 * Functions for setting up fullscreen or hiding the mouse may be called
   from user code outside the application, thus they are kept public.
Doc++
Platform: make viewportEvent() only optional.

Don't do anything to respond to viewport size by default, as the window
has fixed size in most cases anyway (always fullscreen, canvas of fixed
size in browser etc.). Makes the initial implementation requirements
much simpler and shorter.
doc: updated Getting Started guide.