Wrap CMAKE_SHARED_LIBRARY_CXX_FLAGS in quotes.

The variable is empty on some platforms (Windows), would cause issues.
ColladaImporter: fixed "maybe uninitialized warning".

The code is now also more readable.
Fixed visibility macros for plugins used as dependencies.

The symbols weren't exported on mingw32, causing linker errors.
Explicitly specify library types for *TestLib targets.

Use static libraries for plugins which aren't used as dependencies (as
they don't export any symbols anyway), use shared libraries for plugins
used as dependencies (as they already export the symbols and static
libraries cause linker errors on mingw32).
MagnumFontConverter: the test needs OpenGL.
Link plugin dependencies to libraries, not test executables.

The test executables shouldn't need it at all.
Link plugins to their dependencies on Windows.

Fixes linker issues, not present anywhere else.
Cleaned up and unified error and assertion messages.
MagnumFont: don't forget to close on destruction.
No need to make destructors as virtual.

The parent ones are marked already.
Properly update tests after message cleanup.

Followup to 03e81361c979c2f92ed646c2bef1cd615bfe9ba9. I should test
before pushing.
Fix minor documentation errors in FindMagnumPlugins.cmake.
Added PngImporter plugin.

Uses libPNG.
PngImporter: update documentation to mention ImageType::UnsignedShort.
Fixed typo.
Add missing MagnumFontConverter to FindMagnumPlugins.cmake.

Ahem.
New JpegImporter plugin.

Uses libJPEG.
ColladaImporter: deinlined heavy functions, removed redundant `inline`.

Also minor cleanup.
Use `std::` prefix everywhere.
ColladaImporter: minor code reorganization.

Follow order in which the things are declared in header.