[FIX] web: fix overlaping overlays

**Before this commit**
Open some popover then a dialog.
The popover appears in front on the dialog.

Examples:
- Calendar > Click an event (popover opens) > Open command palette
- Open a facet domain editor dialog in any app, then open any model
  field selector popover and open the command palette

**After this commit**
Each first-level child of the overlay container
starts with the same z-index and with a value
that is greater than the higher value we can find
in the bootstrap z-index master list.
This solves the aforementioned overlaping issues.

closes odoo/odoo#140953

X-original-commit: 9682cfa174fb39ec9d9874951ea9ee2884058d96
Signed-off-by: Romain Estievenart (res) <res@odoo.com>
Signed-off-by: Bruno Boi (boi) <boi@odoo.com>

addons/web/static/src/core/overlay/overlay_container.scss

@@ -0,0 +1,7 @@
+.o-overlay-container > * {
+    // This will make sure that each overlay starts with the same z-index.
+    // The chosen value is over the maximum z-index of the bootstrap components.
+    // See: the zindex-stack in _variables.scss
+    position: relative;
+    z-index: $zindex-tooltip + 1;
+}