feature: add support to add multiple instance for a single filter
If i want to filter issues with filter of a multiple value allowed field, named "OS", it contains some of predefined values such as OS1, OS2, OS3...
If i want to filter issues has value OS1 AND OS2, with the feature, I can:
first add Filter "OS", operator "IS", value "OS1".
then add FIlter "OS" again, operator "IS", value "OS2".
This will be useful for lots of usage.
Such as IS OS1 and IS NOT OS2
Contains string1 and not contains string2
it's very useful.