This was originated for #1279 (CLI-only Aseprite) which can be
achieved with LAF_BACKEND=none anyway.
In this way we simplify the development process, and checking for the
availability of the GUI can be done in run-time through App::isGui()
or Context::isUIAvailable().
This is the first attempt to finally implement the require() function
on Lua. The main problem was how to solve conflicts between plugins
that use the same library name. Here we separate each plugin like in a
namespace, so require(name) inside a plugin will save the module in
_LOADED["pluginName/libraryName"] to avoid conflicts with other
libraryName from other plugins.
Instead of storing a Command* we can store the command ID, which it's
less problematic in extreme cases where the Command that is being
referenced is deleted or recreated, i.e. when a plugin is
disabled/re-enabled.
This is a problem introduced with the plugin groups, but now we use a
group to store the list of recent files. With this commit we fixed
some bugs in the impl of menu groups.