Inconsistent text filter behavior

[xusheng6]

Right now, we have multiple widgets that support filtering of the items. However, there behaviors are not consistent. In particular, all widgets that inherit from FilterTarget class, i.e., those in the side bar are doing a plain text match. However, xref and search result is doing a regular expression match. I suggest we unify the behavior, e.g., also migrate FilterTarget to regular expression matching.

[xusheng6]

Also we should document the filtering behavior better so the users can best utilize it.

[xusheng6]

Note he sidebar widgets search is case-insensitive. We should keep the behavior if we switch to regex

[xusheng6]

Related to #3383