entity-manager: separate main.cpp

Create a separate main.cpp for the main function of entity-manager to
facilitate unit testing at a later point. The implementation of
EntityManager (the class) is more clearly separated from entity-manager
(the application).

Tested: Since the code is only moved and otherwise unchanged, nothing to
test.

Change-Id: Iab5dfccd8ec2122abeb5b044a147798f046646fc
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/src/entity_manager/meson.build b/src/entity_manager/meson.build
index 200c9f9..c55a6a8 100644
--- a/src/entity_manager/meson.build
+++ b/src/entity_manager/meson.build
@@ -19,6 +19,7 @@
     'topology.cpp',
     'utils.cpp',
     '../utils.cpp',
+    'main.cpp',
     cpp_args: cpp_args_em,
     dependencies: [
         boost,