Quick "Modal Window" opening leads to js error

When you have some slow js on the page, like 500 records per page and while
they are being loaded try to open a window like pressing "Add" toolbar
button, then you will get JS error about "TB.imgLoader" being undefined.
Here is the patch to fix that.

Also it will be good to disable all toolbar buttons until page is fully
loaded, since working with half loaded javascript could produce strange
error, especially in modal window mode. Here is the patch for that too.