feature: add support to add multiple instance for a single filter

[tainewoo]

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". And apply.

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.

[tainewoo]

Hi @alexandermeindl ,

hope u can consider this, currently the query of core Redmine is too simple, this feature will be useful for all to search issues according to different cases