Rename ThunarLauncher class to ThunarActionManager

Hi there, I have a little patch for you. Only 4654 lines 😄

Sometimes I abbreviated "manager" with "mgr" to make names not too long. Sometimes I thought this really makes sense but I also applied this to function names. Not sure if you like it here too. Just let me know and I make it long again. Also the preprocessor macros are sometimes a bit long... And we have macros like "THUNAR_ACTION_MANAGER_ACTION_OPEN" which have two times the word "ACTION". I also tried to keep names in line vertically so that variables and function arguments look pretty.