Implement new "Add to project" logic

[nyalldawson]

Fixes https://github.com/felt/qgis-plugin-discussion/issues/30

[doorgan]

I have some observations on the design, we'll do a design review pass on a release candidate, so we can tackle the feedback then. In the meantime, I'd like to know how much of the feedback is doable, and how much is limited by the UI toolkit(in particular the selection color, I have the gut feeling it's globally configured so I'd like to confirm that so we can factor that into the design adjustments)

I also wanted to note that we still can open the plugin without a project open, so we'll like to fix that here or in a different PR

Otherwise, the behavior is correct and works as expected, so it looks good to merge after conflict resolution 👍

• The selected color is blue on the plugin, and the + placeholder doesn't have a background color:
  
  Design:
  

• The designs have an outline color in the thumbnails that isn't present in the plugin:
  

• The thumbnails have a different aspect ratio, they're 4:3 in the design, but they're larger horizontally in the plugin. Our thumbnail images source isn't 4:3, so the plugin will need to crop or resize the image if possible

• When you first open the plugin dialog, only the option to create a new map is visible due to the scrolling list being too short. We'll likely want to make it taller so at least 1 or 2 maps are visible without having to manually resize the window. In this case it seems to be caused by the notice on non-compatible layers.
  

• In the dark theme, the UI looks light except for the maps list
  

• The maps list doesn't show the last visited date. We don't provide this on the Felt API so I'll be working on that