Qt plugin system needs some reworking

Qt currently needs to dlopen plugins to find out about the features they support. Change the mechanism to not require this anymore.

Current idea is to embed this information into a section in the ELF file and parse it. Similar for windows in the COFF file.

qplugin.h contains this:

// QT5: should probably remove the entire pattern thing and do the section
// trick for all platforms. for now, keep it and fallback to scan for it.