Expand search options

WIP. See commit logs for more specifics.

See also #11

This MR and set of commits expands the HexDocument API and adds a checkbutton/popover to the search bar to allow for search options.

Currently, "none" (default; exact string matches), regex, case insensitivity, and any combination thereof are supported.

TODO:

• This breaks ABI by expanding public structs. Need to decide to refactor further not to break ABI or live with the ABI break and bump the soname. Not too hot and bothered about it given that probably zero projects actually link to this library outside of ghex. The soname major version has been bumped as a result. The API is not broken. Padding was added to the public structs to prevent future breakage post libgtkhex 4.2.
• I think while we're poking around in findreplace, it would be nice to separate more of the GUI code which is done programmatically into .ui files for ease of maintainability. DONE.