Dropping GCC 4.7 and CMake 2.8.12 support

[mosra]

Even the old and venerable Ubuntu 14.04 (that's currently probably used only on Travis CI) has 4.8 as a default, so I think it's time to drop GCC 4.7. Moreover, Travis has 16.04 now, so it's time to drop CMake 2.8.12 as well.

Things to do:

• check what version of GCC 4.8 has rvalue overloads for this (it was not 4.8.0, probably 4.8.1 ?)
  • check if we can use 4.9 or 5 (no we can't, CentOS is on 4.8.5)
  • make that version as minimal (update docs in all repos)
• remove CORRADE_GCC47_COMPATIBILITY from everywhere and all related workarounds
• update Travis configs to use 4.8 instead of 4.7 (i think they now default to some 5.3, newer than the one that's stock in Ubuntu 14.04)
• verify we can drop CMake 2.8.12 (yes, 14.04 is too old to bother with)
  • check if we can use something newer than 3.0 (no, Debian 8 (Jessie) has 3.0.2) 3.1, 3.0 has a bug
  • make that version as minimal, suggest using prebuilt versions on ancient distros
• remove CMake 2.8.12-specific workarounds in all repos
• remove all CMake policies that get activated in 3.0 3.1 from everywhere (CMP0050 and below) (all of them)
• 3.0 supports message(DEPRECATED), any use for that (except all the deprecated things from 2016 I'm now removing)? nope
• there is a nasty regression in 3.0 breaking the build because Corrade::PluginManager dependency is not propagated upwards, probably need to bump to 3.1 because of that (and make Debian 8 unsupported) ... or is a workaround possible? maybe reverting https://cmake.org/cmake/help/v3.12/policy/CMP0028.html ? nope
  • 3.1 has 64bit prebuilt binaries, so I can clean up travis.yml
  • 3.1 has the CMP0054 property implicit (last one we actually use)
  • I could jump all the way to 3.5, since the next oldest supported version is 3.5.1 in 16.04 (Jessie backports have 3.6) nope, play it safe
    • 3.3 has CROSSCOMPILING_EMULATOR, could be used to avoid dependency on native Corrade for Emscripten (at least) ... which we could support right away because Emscripten needs 3.7 anyway completely orthogonal to this
    • 3.3 has also COMPILE_LANGUAGE generator expression, which could help with making CORRADE_PEDANTIC_FLAGS work correctly for C sources not important, also could have that version-specific
    • 3.5 has imported targets in FindPNG who cares
    • 3.5 shows deprecation warnings by default i don't have any
• be happy because another annoyance got removed 🥃 ✨ 🍻 🎉
• update bootstrap repos as well

This is postponed to after 2018.08 2018.10 to avoid breaking stuff too early. If anyone still needs 4.7, please let me know.

[mosra]

Bootstrap repos updated. Good riddance, old GCC and CMake! 🎉