fix(Main): switcher state and breakpoints between is_mobile

[GeopJr]

fix: #734

To avoid a breakpoint condition hell, let's just wrap it in another stack.

The problem:
We want the main window to show the main view switcher and switcher bar ONLY in narrow mode (is-mobile). To achieve the bottom bar one, is-mobile is bound to its visibility and the main view breakpoint is bound to its reveal property. That way, there are two visibility states. That is not the case for the switcher however, if the switcher is not visible, the headerbar stack cannot set it as the visible child, so both the breakpoint condition and the is-mobile one need to be in sync. That led to the previous very complicated solution.

This time let's just have 2 visibility states for it, like the switcher bar, using two stacks, one for is-mobile and one for the breakpoint.

[GeopJr]

Merging and hoping for the best

[IBBoard]

Works for me on an initial test. It started with the view buttons and they're appearing and disappearing as expected so far 🙂