* Split up Window Layout loading into a loading and application stage
Previously, we were creating UI elements at while we were reading the window-layout.json file.
We now read the window-layout.json file fully first, which results in a
WindowLayout struct which is built up of a list of windows with a list
of tabs with a root node which contains containers and splits.
This WindowLayout can then be applied.
This will enable PRs like #1940 to start Chatterino with Window Layouts
that aren't defined in a json file.
This commit has deprecated loading of v1 window layouts (we're now on v2). If a v1 window layout is there, it will just be ignored and Chatterino will boot up as if it did not have a window layout at all, and on save that old window layout will be gone.
* Fix compile error for mac
