ui: stack manager

[greatgitsby]

move bespoke panel switching and bespoke modal handling into a single stack interface

likely going to break out changes from this, there are a few fundamental changes in this PR:

• set_modal_overlay no longer needed, use stack
• couple bugs found in keyboard renderer
• "modal" operated differently than normal "widget" as it had a return code and handled it uniquely. want to make this a general feature of Widget.
• i hate callback invoking pop, it should be simpler than this (dismiss should auto-handle pop if no action was taken)

[github-actions]

raylib UI Preview

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[sshane]

BTW a requirement for this kind of PR should be: easy to render the previous widget behind if swiping away