added a customizable plugin system

- the base LUA functionality has been implemented as a mandatory plugin
- the jtacautolase functionality has been implemented as a plugin
- added a VEAF framework plugin

The plugins have GUI elements in the Settings window.

plugin/__init__.py

@@ -0,0 +1,10 @@
+from .base_plugin import BasePlugin
+from .veaf_plugin import VeafPlugin
+from .jtacautolase_plugin import JtacAutolasePlugin
+from .liberation_plugin import LiberationPlugin
+
+INSTALLED_PLUGINS=[
+    VeafPlugin(), 
+    JtacAutolasePlugin(), 
+    LiberationPlugin()
+    ]