[LLDB][GUI] Add initial searcher support

This patch adds a new type of reusable UI components. Searcher Windows
contain a text field to enter a search keyword and a list of scrollable
matches are presented. The target match can be selected and executed
which invokes a user callback to do something with the match.

This patch also adds one searcher delegate, which wraps the common
command completion searchers for simple use cases.

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D108545