core: fixes and changes to temp files

* fix delete_temp_files() deleting in EXEDIR but temp_filename() was
  creating temp files in USERDIR
* make delete_temp_files() public so it can be used by plugins
* add optional `dir` parameter to both delete_temp_files() and
  temp_filename() to allow specifying a different directory, this is
  for example useful when generting markdown previews, the temp file
  should be generated in the project dir in case the readme references
  images that are relative to it, so the web browser can find them.

data/core/init.lua

@@ -809,17 +809,19 @@ local temp_uid = math.floor(system.get_time() * 1000) % 0xffffffff
 local temp_file_prefix = string.format(".lite_temp_%08x", tonumber(temp_uid))
 local temp_file_counter = 0
 
-local function delete_temp_files()
+function core.delete_temp_files(dir)
-  for _, filename in ipairs(system.list_dir(EXEDIR)) do
+  dir = type(dir) == "string" and common.normalize_path(dir) or USERDIR
+  for _, filename in ipairs(system.list_dir(dir)) do
     if filename:find(temp_file_prefix, 1, true) == 1 then
-      os.remove(EXEDIR .. PATHSEP .. filename)
+      os.remove(dir .. PATHSEP .. filename)
     end
   end
 end
 
-function core.temp_filename(ext)
+function core.temp_filename(ext, dir)
+  dir = type(dir) == "string" and common.normalize_path(dir) or USERDIR
   temp_file_counter = temp_file_counter + 1
-  return USERDIR .. PATHSEP .. temp_file_prefix
+  return dir .. PATHSEP .. temp_file_prefix
       .. string.format("%06x", temp_file_counter) .. (ext or "")
 end
 
@@ -834,7 +836,7 @@ end
 
 local function quit_with_function(quit_fn, force)
   if force then
-    delete_temp_files()
+    core.delete_temp_files()
     core.on_quit_project()
     save_session()
     quit_fn()