Do not allow to open one file in multiple editors/tabs

[pellicus]

I have multiple tabs on the same file X , let say t0 , t1 , t2
if i modify and save working on t1 i get a pop up saying me that the file in t0 is modified from an external
editor , after that i get another pop up for tab t2.

double clicking on the same file open multiple tabs

proposal:

• double clicking on a already opened file just select the right tab.
• adding a menu command to duplicate the current tab. (even if it's rarely useful)
• avoid the pop up in case of saving from textosaurus (tracking the saved files in a map to clean after everything is saved and tabs are finally synced)

[martinrotter]

I added detection of already opened files which is now used to open file in existing tab if there is one. Please, test.