Add HdySideBar
Several applications have a horizontal panel split where the left-most panel is more of an alternative view on the data than an upper level in the data hierarchy. These tend to be implemented as sidebars and their split don't propagate through the titlebar, and they can sometimes be manually hideable. The sidebar isn't an overlay on the main view and sides beside it.
Notable examples are:
- Files's (nautilus) sidebar which is a more a shortcuts list than an upper level
- Document Viewer's (Evince) sidebar which is a smaller version of the main view useful for quick navigation
- Usage's performance page sidebar which contians shortcuts to the different sections of the page
We want the sidebar to be optionally visible when the window is large, but to be automatically hidden and able to appear as an overlay when the window is small. For that, a leaflet doesn't work well because it would make the sidebar widget cover completely the main view, as if it was more important. We want to keep implying this sidebar widget is less important than the view, for that the sidebar shouldntt completely cover it, so we need a new widget for that specific use case.
We liekly want to support having sidebars at both ends, maybe also on top and below. Or just at the ends (with orientation) and two could be nested to have sidebars all around the main view.