refactor: collapse speed-dial action source abstraction

Only one action source ever existed, so the
IActionSource interface and ScriptActionSource
are gone. ActionRegistry now subscribes to the
plugin loader and enumerates actions directly
in init() - no polymorphism for one impl.

Replace the opaque FNV-hash SpeedDialActionId
with a readable composed id of the form
prefix:title:source_key. Split AppAction's
single source field into source_key (stable
identity, e.g. plugin_key) and source_name
(display), so identity and display no longer
share one field.

tests/slic3rutils/CMakeLists.txt

@@ -8,7 +8,6 @@ add_executable(${_TEST_NAME}_tests
     test_plugin_lifecycle.cpp
     test_slicing_pipeline_bindings.cpp
     test_plugin_sort.cpp
-    test_speed_dial_action_id.cpp
     )
 
 if (MSVC)