overlay: OBS control window via WebSocket API (#773)

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

## Description of change
Adds an overlay window widget for controlling OBS over WebSocket.

Add new lines to the FPS widget that shows recording / streaming timers.

Show notifications for major state changes (stream started/stopped,
recording started/paused/resumed/stopped)

## Testing

src/spice2x/overlay/overlay.h

@@ -77,6 +77,7 @@ namespace overlay {
         Window *window_camera = nullptr;
         Window *window_sub = nullptr;
         Window *window_log = nullptr;
+        Window *window_obs = nullptr;
 
         // not part of `windows`: drawn/updated on the persistent layer (like
         // notifications), independent of the overlay's active state and input gates.