overlay: refactor overlay layering (#739)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change

Create three distinct layers for the overlay:

1. Bottommost persistent layer - non-interactable layer that is always
on. This was only for notifications, but now the FPS widget lives here
as well.
2. Overlay windows layer - most interactable windows go here.
3. Topmost main menu - this is reserved for the main menu (escape key)
and this is a modal dialog that occludes the layers below.

Why? 

- `toggle overlay` behavior with FPS widget *also* toggling on/off was a
bit confusing (now they're two separate keys)
- FPS widget is popular, but it caused the entire overlay to be active,
which affects how input is handled
- the main menu being a standalone window was a little awkward (now it's
a modal)

## Testing

src/spice2x/overlay/window.h

@@ -24,15 +24,26 @@ namespace overlay {
         void toggle_active();
         void set_active(bool active);
         bool get_active();
+
+        // raise this window to the top of the z-order on its next build. only
+        // call in response to an explicit user action (keyboard toggle, UI
+        // button) - not for auto-show / programmatic shows.
+        void bring_to_front();
     protected:
 
         // state
         SpiceOverlay *overlay;
         bool active = false;
+        // set when the window transitions to visible so build() raises it to the
+        // top of the z-order on the next frame
+        bool request_focus = false;
         std::vector<Window*> children;
 
         // settings
         bool remove_window_padding = false;
+        // custom inner window padding, applied before Begin() (preserving border /
+        // rounding) when x >= 0; ignored if remove_window_padding is set
+        ImVec2 window_padding = ImVec2(-1, -1);
         bool draws_window = true;
         ImGuiSizeCallback resize_callback = nullptr;
         std::string title = "Title";
@@ -41,6 +52,8 @@ namespace overlay {
         bool toggle_button_state = false;
 
         // init settings
+        // calculate_initial_window() runs only once; results feed ImGuiCond_Once
+        bool initial_window_calculated = false;
         ImVec2 init_pos = ImVec2(0, 0);
         ImVec2 init_size = ImVec2(0, 0);
         ImVec2 size_min = ImVec2(0, 0);