Fix: inexistent -> nonexistent.

No publicly visible change, tests only.
Utility: open resource configuration files read-only.

The variable was const already, but this is even more hardened.
Utility: improve and test Resource compiler error reporting.
PluginManager: verify that instance is returned only from proper manager.

Currently fails. How could I forget to check this obviously dangerous
error?!
PluginManager: return instance only from proper manager.

The tests now pass again.
examples: fix compilation when Corrade is installed w/o WITH_FIND_MODULE.

The examples expected FindCorrade.cmake to be installed in system
location.
doc: crosslink examples with CMake macros documentation.

Also removed obsolete information from PluginManager example.
dox: fixed PluginManager example code listings.
examples: use Utility::Arguments in PluginManager example.

Makes everything more shiny and whatnot.