Instead of requiring users to enable the core c++ plugin 'Coordinate Capture'
in order to copy coordinates for a map point, instead add this functionality
to a map canvas right click context menu.
Now, when compatible map tools are activated (pan, zoom, select by rect), right clicking
on the canvas shows a context menu with a "Copy Coordinate" submenu. The
submenu shows options for copying the coordinate in the map CRS, WGS84 or
a custom preset CRS.
Also adds API to allow 3rd party QgsMapTool subclasses to implement their own
context menus which include this Copy Coordinate action.
For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
