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/CMakeLists.txt

@@ -605,10 +605,15 @@ set(SOURCE_FILES ${SOURCE_FILES}
         overlay/windows/keypad.cpp
         overlay/windows/log.cpp
         overlay/windows/midi.cpp
+        overlay/windows/obs.cpp
+        overlay/windows/obs_websocket.cpp
         overlay/windows/patch_manager.cpp
         overlay/windows/popn_sub.cpp
         overlay/windows/wnd_manager.cpp
 
+        # external
+        external/easywsclient/easywsclient.cpp
+
         # rawinput
         rawinput/rawinput.cpp
         rawinput/sextet.cpp