Support Design Blocks (reuse library) for schematics (lp:#1797683)

Original report created by Thomas Pointhuber (pointhi)

It would be nice to have the possibility of inserting parts of a schematic
or even whole hierarchical-sheets from a special purpose library.

Basically, this would allow simplifying the reuse of existing modules like
a step-down regulator or in my case an Arduino-compatible hierarchical
sheet.

Somethings similar (they needed it because of lack of hierarchical
schematics :P):

https://www.autodesk.com/products/eagle/blog/whats-new-in-autodesk-eagle-
modular-design-blocks/
https://www.youtube.com/watch?v=yfsENn6Gq3M

# Version Information

Application: kicad
Version: (6.0.0-rc1-dev-899-g1bce69b9f), debug build
Libraries:
    wxWidgets 3.1.1
    libcurl/7.61.1 OpenSSL/1.1.1 zlib/1.2.11 libidn2/2.0.5 libpsl/0.20.2
(+libidn2/2.0.4) libssh2/1.8.0 nghttp2/1.33.0
Platform: Linux 4.18.12-arch1-1-ARCH x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.1.1 (wchar_t,wx containers) GTK+ 2.24
    Boost: 1.68.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.61.1
    Compiler: Clang 7.0.0 with C++ ABI 1002

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_WXPYTHON=OFF
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON

Original tags: eeschema