Code reorganization.
Doc++
Provide MAGNUM_TARGET_WEBGL also in CMake.
DebugTools: defaulted destructor implementation in source files.

To avoid errors about deleting incomplete when destructing Resource
instances. Spotted by Clang.
Updated toolchains submodule.
package: adapted Emscripten PKGBUILD to emscripten-git AUR package.
Math: added Range::data().

The main side-effect of this is that the data layout is now really
specified, making it useful for various glGet*(GL_*_RANGE) calls.
Added Context::detectedDriver().

A general way to detect drivers, which can be later used for applying
driver-specific workarounds. Currently used for disabling
ARB_explicit_uniform_location on AMD drivers.
Detecting whether ANGLE is used as GLES2 implementation.
Shaders: fix compilation of MeshVisualizer on WebGL and ANGLE.

WebGL mandates that array subscription is done with constant expression,
ANGLE too (but I think that has also something to do with D3D9
limitations). This is however allowed by OpenGL ES 2.0 specification, so
enabling the workaround only for WebGL and ANGLE (i.e., this won't apply
to Native Client using native GL drivers).
Oh Doxygen you so predictable.
Text: renamed configure.h.cmake to fontconverterConfigure.h.cmake.

Reason is that the configure file is not for whole Text library, but
only for the magnum-fontconverter utility. Rename it so it is more
clear.
Text: unified help message for plugin names for all utilities.
Text: actually make use of plugin-dir parameter in fontconverter utility.

What the hell.
TextureTools: configurable plugin dir for distancefieldconverter utility.

Similarly to magnum-fontconverter utility.
Minor code reorganization.
Add back the missing MAGNUM_PLUGINS_INSTALL_DIR variable.

Caused the utilities to have empty plugin dir by default. Broken since
the debug/release library separation.
Proper default plugin dir for debug on multi-configuration buildsystems.

Similar logic to what we expose to the outside world.
doc: make section headers in class/function docs have sane size.

Similar reasoning as in mosra/corrade@93b8308bc6a4def9d4564a6cea236cabe54fe063.
Improvements in utility help texts.