plugins: renamed plugins-common.h to plugin-state.h

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: renamed plugins-manager.h to plugin-manager.h

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: renamed PluginsManager to PluginManager

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: remove plugins_MOC_SRCS from CMakeLists

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: renamed directory to plugin

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: moved activation code to separate directory

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>