Add built-in search in code function

[luizzeroxis]

Description

Added a search in code option (Ctrl+Shift+F or in the Search menu) that's meant to replace the Search.csx script. It's faster to open, has that shortcut, and has all the options in one screen. Related issues: #545, #942, #1814

Caveats

There's definitely improvements to be made

Notes

Mostly copied from Search.csx ~~and ClickableTextOutput~~, so it's a little messy. Using LoaderDialog sucks, not sure if that should be improved or there should be a new loading bar inside the window itself. ~~That rich text box should probably be a table or list or something.~~ Searches should be cancellable. Also I'm not sure how to deal with all those threads being used, might not be 100% thread-safe.