create actor as abstracted game_object

[noah1510]

This is or rather will be one of the most important features since it is the class almost everything that you see on the display is based on.

Unlike the game_object, it should own a pointer to the used shader, unless it is the default shader, in that case the pointer should be nulltpr and the default_shader added to the active world should be used.
*The actor always uses the default shader!~

Things that are still missing:

• implement the Actor class as abstracted game_object
• collision events #18
• updates to the testgame to only use actors

[noah1510]

While 9385ecd updated the testgame with actors there are still a few issues to be fixed before merging the code into master

[noah1510]

it seems there are some problems with the textures at the moment, after that is fixed 0.0.13 0.1.1 will be released.