modules: updated FindCorrade.cmake and FindMagnum.cmake.
package: removed superfluous character.
modules: updated FindCorrade.cmake and FindMagnum.cmake.
Install plugins in debug build to different location.

Makes it possible to have both debug and release plugins installed. If
both debug and release libraries are present when finding the package,
proper version is used based on what configuration is used in depending
project.
package: added release PKGBUILD.

It compiles and installs both debug and release libraries.
doc: removed info about internal CMake variables.

Keep the docs only in FindMagnumPlugins.cmake.
package: strip binaries in CI build.
Prefix plugin test libraries with Magnum*.

They need to be installed into possibly system-wide location on Windows
and thus we need to avoid name clashes (or at least explicitly show that
e.g. PngImporterTestLib.dll belongs to Magnum and is not any OMG virus).
Merge branch 'master' into compatibility