generalize the components for appending controls to the input/textareas

[joshlong]

we do this in three places in the UI:

• WritingTools (for AI tools to edit texts)
• previewing secrets in the settings screen
• soon, we'll do somethign like this in the Markdown preview editor.

some things strike me as generalizable:

• an icon on the right of the input
• potentially: a popunder div that shows up beneath the text offering controls
• updating a dom binding once the input has changed

at the very least there's an icon and a need for consistently updating a bidning. the popunder is a different thing besides.

what if a panel has access to more than one of these wrapper components? how should the icons stack themselves? how shoudl the popunders play together? shoudl they all hide themselves so that one might show themselves? should they stack? does this mean we should have an event that subcomponents in the pupunder would need to respect? eg, a uniform 'x' to close the popunder?

[joshlong]

this is done. we now have machinery for contributing panels that act on the currently selected text input or textarea. first up? a markdown preview!