Add an extension point for Lua scripts

[supertobi]

Add an extension point for Lua scripts, then you could build the scripts as a
flatpak extension including the binaries some scripts use.

[hfiguiere]

See also issue #35 for general Lua support.

[hfiguiere]

Do you happen to have some of these preferred extensions you'd want to see? So that when there is an extension point there is something to use it for.

Thanks,

[supertobi]

The official Lua repro is her:
https://github.com/darktable-org/lua-scripts

As a simple example you could use:
examples/hello_world.lua

The two that I'd like to have are:
https://github.com/darktable-org/lua-scripts/blob/master/contrib/geoToolbox.lua
https://github.com/darktable-org/lua-scripts/blob/master/contrib/kml_export.lua

[hfiguiere]

The script manager should be available when we merge #66 (ie soon, today). Let us know if there is still something missing.