Replace Core Dialog with a custom sidebar solution

Problem/Motivation

Currently Homebox (3.x) uses the core dialog component with some custom styles for its sidebar (which contains the addable portlets).

The problem is that some admin themes, and probably other modules, extend these styles so that they break the Homebox sidebar.

The "Offcanvas" variant of the Core Dialog component is more robust to this problem, but this is achieved by a very hard CSS reset, which makes it very, very difficult to add any styling to the content.

Steps to reproduce

Proposed resolution

Build a custom Homebox sidebar.

Remaining tasks

User interface changes

API changes

Data model changes