It looks like this is an undocumented change that happened between 7.7.1 and 7.8 (if this is in the 7.8 release notes, I don't see it). AFAIK, there is no way to change the toolbar options in the Desktop modeler. Seems to me like this may be the result of trying to make the desktop and web modeler interfaces a bit more similar (see the widget toolbox in the web modeler), but perhaps someone from Mendix can add some more info to this thread.
Exactly what Mike says, I checked the release notes as well and didn't find it. Didn't notice it earlier on as well, I thought I had the options available in 7.8 before because I didn't notice this before but ok. Was still testing in 7.8 to find out if you could alter this behaviour. Removing 7.8 and re-installing didn't fix it either.
And it's not only the toolbar which changed, when you click right mouse you have to select add widget and then you can select one from the popup. Right click mouse and then quickly selecting the correct widget isn't working any longer so now I have to do 2 steps extra, click add widget, click the correct widget, click to put it on the correct place. For me personally I would prefer to have the option to choose wich way I want to work.