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.
ColladaImporter: use const reference instead of copying.

Probably forgotten `&`, because at the time this code was written I
wasn't as big const fascist as I am now.
ColladaImporter: avoid needless string copying.

Use std::move() and emplace instead of insert where possible.
ColladaImporter: don't use `Corrade::` prefix when not needed.
ColladaImporter: use XPath's starts-with() instead of substring comparison.

I still hate this XMess, though.
ColladaImporter: no need to use parentheses here.
ColladaImporter: test also all *Name() functions.

sceneForName() is not implemented, thus the test fails.
ColladaImporter: add missing sceneForName() implementation.
ColladaImporter: use default string constructor instead of "".

Might save one unneeded allocation.
ColladaImporter: texture importing support.

Texture integration with materials will be done later.
ColladaImporter: support for textured Phong material.