[TxMt] Option to disable automatic tab creation

[Edmundo Ortega]

We've talked about this before, but I don't think anything became of 
it. Here's the idea:

Create a preference so that new tabs are not created for every item 
clicked in the project drawer. To create a tab you would have to 
shift-command-click or something like that. Without the modified click, 
the current file would open either in a special "current item" tab on 
the left, that would simply be replaced by whichever item is clicked on 
next.

This is similar to Safari's tabbed page behavior.

For super-organized people that remember to close all the tabs they 
don't really need, they can choose not use the preference and retain 
the current behavior.

For people like me that quickly reference other files and then forget 
to close them, this would be a nice way to avoid unnecessary tab 
clutter.

Ed