Add a static build on the CIs

[mosra]

Not so easy, in fact. A HELLISH NIGHTMARE.

• Find modules need to expose MAGNUM_<COMPONENT>_BUILD_STATIC so user code can find plugins and then decide based on whether the plugins are static or not
  • The changes in the modules need to get commited to magnum and magnum-plugins
  • Similar changes done for FindMagnumIntegration, FindMagnumExtras
  • Figure out how to handle this for CMake subprojects (for a future-proof solution the plugins need to tell the configure.h location to the find module -- something like done with _MAGNUM_CONFIGURE_FILE from inside Magnum, putting that as an internal cache entry, then the find module has to find the file instead of assuming its location and parse it even if the target already exist)
  • The Mac build doesn't actually work for some reason, and the AnyImageImporter isn't built as static
• Need to figure out an actual solution for the case where plugins are dynamic but libraries static -- in that case whitelisted Array deleters get duplicated in the executable and in the plugin and then obviously the whitelisting doesn't work and asserts (which is correct, as the deleter would be a dangling pointer after plugin unload). Solve this with yet another "globals shared across static libs" ugly workaround? Sigh.