Moved DefaultShortcutAttribute and RegisterShortcutsComponent to Actions folder.
Refactored action registration. ReplaceActionHandlerManager overrides default actions by usage of ReplaceActionHandlerAttribute (currently used by Run/DebugUnitTestContextAction). DefaultBindingManager assigns shortcut specified by DefaultBindingAttribute, if neither the command nor the binding is already in use (currently used by GotoRelatedTypesAction). Closes #18.
Added lifetime management for default bindings.