Handle WITH_* options for utilities better.

I.e., don't offer them on platforms which don't support them.
TextureTools: renamed magnum-distancefield to magnum-distancefieldconverter.
doc: removed absolutely wrong description, added TODO.
Text: added magnum-fontconverter utility.

Again borrowed from Push the Box, modified and made slightly more
configurable. Will write proper docs for the utilities when I have some
more time.