crecord: add a handlesearch function This function sets up some of the UI, such as getting the search string from the user and displaying results or their absence.

crecord: add a showsearch function This function takes a regex and searches either forward or backward, moving the current item to the found item, if any, and unfolding the relevant context.

crecord: add a default regex to curseschunkselector Whether there is a regex to search or not will affect if we can find the next or the previous search hit.

crecord: add `content` properties to all nodes In order to have a unified API of what can be searched, let's provide a `content` property to each node type. This way we can search filenames, context headers (e.g. containing function names, if deducible from patch context) or changed lines themselves.

crecord: update uiheader docstring There's no need to move anything to patch.py. The uiheader class only has methods relevant to crecord and overrides __getattr__ in order to use `patch.header` objects as a sort of mixin.

crecord: add skipfolded param to previtem This just simplifies the API a bit so it matches `nextitem` and I can handle both nextitem and previtem symmetrically.