Mixed fixes

[jjuhl]

Just a few small cleanups/fixes.

[texus]

The BackendFontFactory::createFont(), BackendRenderer::createText() and BackendRenderer::createTexture() virtual functions can't be made const for backwards compatibility. They are base classes for custom backends. I've added a note to add the const if I ever start working on TGUI 2.

While the contains() function is better, that part of the code is a test for the count() function 😄

[jjuhl]

Makes perfect sense :)
I've dropped those two commits and rebased on the latest 1.x branch.

[jjuhl]

Rescued a few commits from the include-cleanup-part1 branch/PR as we discussed.