refactoring and ida90 support

[yeggor]

This PR includes the following improvements:

• refactoring of plugin and loader code
• IDA 90 support (plugin and loader are currently tested with RC1 beta version and latest SDK available at my.hex-rays.com)

To build with idasdk84:

python3 build.py build-plugin ~/sdk/idasdk84 --hexrays_sdk ~/sdk/hexrays_sdk
python3 build.py build-loader ~/sdk/idasdk84

# or

python3 build.py build-everything ~/sdk/idasdk84 --hexrays_sdk ~/sdk/hexrays_sdk

To build with idasdk90:

python3 build.py build-plugin ~/sdk/idasdk90 --hexrays_sdk ~/sdk/idasdk90
python3 build.py build-loader ~/sdk/idasdk90

# or

python3 build.py build-plugin ~/sdk/idasdk90 --hexrays_sdk ~/sdk/idasdk90

When installing efiXplorer for IDA 9.0, only one library file should be used, e.g.:

ln -s $(pwd)/efiXplorer/build/efiXplorer64.dylib ~/.idapro/plugins/efiXplorer.dylib