Unify confirmation popups into popup framework

[neiljp]

What does this PR do, and why?

This incrementally improves the popup code until a point where the confirmation popups can be handled in the same way as others and improved upon:

• extract the border styling from core.py and into a new PopUpFrame class in view.py
• extend PopUpFrame to handle title-less cases (ie. for confirmation popups)
• simplify confirmation popups to not support locations other than center, easing integration
• migrate ConfirmationPopUpView from an Overlay to a Frame, and other changes to enable use of show_pop_up just like other popups

Other than simplifying the code, this also results in confirmation popups acquiring a solid edge, as with regular popups.

Outstanding aspect(s)

• [ ] Last commit needs more consideration
• [ ] Nesting of popups into frame suggests frame could be an urwid container/decorator class
• [ ] Confirm ability to style popup borders
• [ ] Select appropriate popup borders for different confirmation popups?

External discussion & connections

• [ ] Discussed in #zulip-terminal in topic
• [ ] Fully fixes #
• [ ] Partially fixes issue #
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [ ] Merge will enable work on #

How did you test this?

• [ ] Manually - Behavioral changes
• [ ] Manually - Visual changes
• [ ] Adapting existing automated tests
• [ ] Adding automated tests for new behavior (or missing tests)
• [ ] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [ ] It is a minimal coherent idea
• [ ] It has a commit summary following the documented style (title & body)
• [ ] It has a commit summary describing the motivation and reasoning for the change
• [ ] It individually passes linting and tests
• [ ] It contains test additions for any new behavior
• [ ] It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes