fix(wm): skip layout calc for empty workspaces

While investigating issue #2 I was able to reproduce it and view the
panic that causes the komorebi process to become non-responsive.

When switching to a columnar layout (which is the default for the 2nd
workspace in the sample ahk config), there is the possibility to cause a
divide by zero panic if the len passed to Layout::calculate is 0.

I have remedied this by changing the type of len from usize to
NonZeroUsize, and also by ensuring that Layout::calculate is only called
from within the komorebi crate if the workspace has at least one
container.

While moving containers around I also noticed that creating a new
container for a window may also cause a panic if focused_idx + 1 is
greater than the length of the VecDeque of containers, so this was
addressed by pushing to the back of the VecDeque in that case.

re #2

komorebi-core/src/layout.rs

@@ -1,3 +1,5 @@
+use std::num::NonZeroUsize;
+
 use clap::Clap;
 use serde::Deserialize;
 use serde::Serialize;
@@ -127,11 +129,12 @@ impl Layout {
     pub fn calculate(
         &self,
         area: &Rect,
-        len: usize,
+        len: NonZeroUsize,
         container_padding: Option<i32>,
         layout_flip: Option<LayoutFlip>,
         resize_dimensions: &[Option<Rect>],
     ) -> Vec<Rect> {
+        let len = usize::from(len);
         let mut dimensions = match self {
             Layout::BSP => recursive_fibonacci(
                 0,