resources: .desktop inode/directory mime type

This allows associating lite-xl on linux with folders for the open with
option of graphical file browsers.

resources/linux/org.lite_xl.lite_xl.desktop

@@ -7,4 +7,4 @@ Icon=lite-xl
 Terminal=false
 StartupWMClass=lite-xl
 Categories=Development;IDE;
-MimeType=text/plain;
+MimeType=text/plain;inode/directory;