When creating a copy of CONTEXT_MENU, always a CONTEXT_MENU instance was
constructed, whereas an inherited type should be used. Solved with
CONTEXT_MENU::create() that has to be overridden in inheriting classes.
Event & update handlers are now virtual functions, instead of setting
the handlers with Set{Event,Update}Handler().
* Use only double to store zoom levels in class BASE_SCREEN (int type is not working for eeschema or page layout editor) (this fix Bug #1450551, Assertion failed on wxRound() when zooming)
* Fix crash in all apps but Pcbnew when a grid or a zoom was selected from the popup menu.
* Modify default hotkeys for Next Grid and Previous Grid: the initial key was a dead key, not suitable as hotkey. the 'N' and Shift N keys are used instead (with a change for unfill zone hotkey)
* Modify code in EDA_DRAW_FRAME::SetNextGrid() and EDA_DRAW_FRAME::SetPreviousGrid() to make them compatible with other editors than the board editor.