Adding/remove a tag freezes GTG

[Izidor Matušov]

Steps to reproduce:
1) Select all your tasks (it is reproducable with my 250 tasks or even less)
2) Press <ctrl>+t which opens Modify tags dialog
3) add or remove a tag

Actual result: GTG freezes for couple of seconds
Expected result: GTG shouldn't freeze

Adding or removing a tag shouldn't be an expensive operation and we are probably doing something wrong. If the problem is not solvable, it should be done in background with progressbar dialog