Split plugin classes and derive AIFilePicker from BasicPluginBase (part 1).

This commit contains all changes, except those in indra/media_plugins
(which was renamed to indra/plugins) and indra/llplugin.

However, it does contain the (new) file
indra/plugins/filepicker/basic_plugin_filepicker.cpp

indra/cmake/BasicPluginBase.cmake

@@ -0,0 +1,8 @@
+# -*- cmake -*-
+
+
+set(BASIC_PLUGIN_BASE_INCLUDE_DIRS
+    ${LIBS_OPEN_DIR}/plugins/base_basic/
+    )
+
+set(BASIC_PLUGIN_BASE_LIBRARIES basic_plugin_base)