[LLDB][GUI] Add Breakpoints window

This patch adds a breakpoints window that lists all breakpoints and
breakpoints locations. The window is implemented as a tree, where the
first level is the breakpoints and the second level is breakpoints
locations.

The tree delegate was hardcoded to only draw when there is a process,
which is not necessary for breakpoints, so the relevant logic was
abstracted in the TreeDelegateShouldDraw method.

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D107386