Add the N/Shift+N hotkeys to the eeschema hotkey file.
Also adjust the COMMON_TOOLS::GridNext/Prev to work when
the screen's grid definitions do not have consecutive command
IDs (they do not in eeschema).
This is related to lp:1811018 (but doesn't constitute a fix).
* https://bugs.launchpad.net/kicad/+bug/1811018