Implement lazy loading of directories

When the number of files in a project directory is above the max
limit switch back to a mechanism to read directory content only
when the corresponding folder is expanded in the treeview.

When the command core:find-file is invoked the command core:open-file
is executed instead because the complete list of the project's
files is not available.

When a project search is done we search through all the files within
the project dir without indexing them.

Address issues #217 #203 #183.

data/core/commands/core.lua

@@ -66,6 +66,9 @@ command.add(nil, {
   end,
 
   ["core:find-file"] = function()
+    if core.project_files_limit then
+      return command.perform "core:open-file"
+    end
     local files = {}
     for dir, item in core.get_project_files() do
       if item.type == "file" then