and logic for activating a custom map tool and ensuring it can
only be enabled in the right circumstances to QGIS app

If a plugin has to do this, it's a nightmare of code and hacks (partly
because of the number of changing circumstances it needs to respond
to, and partly because a lot of the useful functions available
for handling this behavior is locked away in private methods
in qgisapp.cpp)

So instead make an abstract base class for map tool handlers and
an iface method for register/unregistering them.

From the dox:

An abstract base class for map tool handlers which automatically handle all the necessary
logic for toggling the map tool and enabling/disabling the associated action
when the QGIS application is in a state permissible for the tool.

Creating these handlers avoids a lot of complex setup code and manual connections
which are otherwise necessary to ensure that a map tool is correctly activated and
deactivated when the state of the QGIS application changes (e.g. when the active
layer is changed, when edit modes are toggled, when other map tools are switched
to, etc).

- ### Example

\code{.py}
  class MyMapTool(QgsMapTool):
     ...

  class MyMapToolHandler(QgsAbstractMapToolHandler):

     def __init__(self, tool, action):
         super().__init__(tool, action)

     def isCompatibleWithLayer(self, layer, context):
         # this tool can only be activated when an editable vector layer is selected
         return isinstance(layer, QgsVectorLayer) and layer.isEditable()

  my_tool = MyMapTool()
  my_action = QAction('My Map Tool')

  my_handler = MyMapToolHandler(my_tool, my_action)
  iface.registerMapToolHandler(my_handler)
\endcode