Context-specific grids for Pcbnew

Spawned from https://bugs.launchpad.net/kicad/+bug/1785881

Other EDA packages have multiple configurable grids that switch automatically depending on what is being moved/what tool you're using, e.g.
 - Part (footprints): Primary and secondary, accessible through a modifier key, mounting type or pin count threshold etc.
 - Route: separate tracks and vias grids.
 - Other: Drawing (graphics, zones?), jumpers and test points etc.

Each one configurable with their own rotation and offset from the origin.

It would be nice if these could be stored in the project and imported from other projects.

IIRC Altium only differentiates between components and non-components, but they have a priority based system [1].

[1] https://www.altium.com/documentation/18.0/display/ADES/((PCB+Grids+System))_AD#!PCBGridsSystem-User-DefinableGrids

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Application: KiCad
Version: (5.1.0-1524-ga0eaff289), debug build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.54.0 LibreSSL/2.6.5 zlib/1.2.11 nghttp2/1.24.1
Platform: Mac OS X (Darwin 18.7.0 x86_64), 64 bit, Little endian, wxMac
Build Info:
    wxWidgets: 3.0.4 (wchar_t,STL containers,compatible with 2.8)
    Boost: 1.70.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.54.0
    Compiler: Clang 10.0.1 with C++ ABI 1002

Build settings:
    KICAD_SCRIPTING=OFF
    KICAD_SCRIPTING_MODULES=OFF
    KICAD_SCRIPTING_PYTHON3=OFF
    KICAD_SCRIPTING_WXPYTHON=OFF
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
    KICAD_SCRIPTING_ACTION_MENU=OFF
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON
    KICAD_STDLIB_DEBUG=OFF
    KICAD_STDLIB_LIGHT_DEBUG=OFF
    KICAD_SANITIZE=OFF