Generalize and rename overlays as "floating menus"

Progress towards #135.

client/web/src/App.vue

@@ -26,7 +26,7 @@
 	// TODO: Replace with CSS color() function to calculate alpha when browsers support it
 	// See https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/color() and https://caniuse.com/css-color-function
 	// E6 = 90% alpha
-	--popover-opacity-color-2-mildblack: #222222e6;
+	--floating-menu-opacity-color-2-mildblack: #222222e6;
 }
 
 html,
@@ -54,7 +54,7 @@ img {
 }
 
 // For placeholder messages (remove eventually)
-.popover {
+.floating-menu {
 	h1,
 	h2,
 	h3,