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/imgui/extensions.cpp

@@ -1,4 +1,5 @@
 #include "extensions.h"
+#include <algorithm>
 #include <cmath>
 
 #include "external/imgui/imgui.h"
@@ -161,6 +162,19 @@ namespace ImGui {
         return clicked;
     }
 
+    bool ColoredButton(const char* label, const ImVec4& base, const ImVec2& size) {
+        const auto brighten = [](const ImVec4& c, float d) {
+            return ImVec4((std::min)(c.x + d, 1.0f), (std::min)(c.y + d, 1.0f),
+                          (std::min)(c.z + d, 1.0f), c.w);
+        };
+        ImGui::PushStyleColor(ImGuiCol_Button, base);
+        ImGui::PushStyleColor(ImGuiCol_ButtonHovered, brighten(base, 0.12f));
+        ImGui::PushStyleColor(ImGuiCol_ButtonActive, brighten(base, 0.22f));
+        const bool clicked = ImGui::Button(label, size);
+        ImGui::PopStyleColor(3);
+        return clicked;
+    }
+
     bool ClearButton(const std::string& tooltip) {
         ImGui::PushID(tooltip.c_str());
         // same colors as a checkbox

src/spice2x/overlay/imgui/extensions.h

@@ -22,6 +22,11 @@ namespace ImGui {
     bool ClearButton(const std::string& tooltip);
     void HighlightTableRowOnHover();
 
+    // a Button with the given base fill color; the hovered/active shades are
+    // derived by brightening the base. size defaults to auto (fit the label).
+    bool ColoredButton(const char* label, const ImVec4& base,
+            const ImVec2& size = ImVec2(0, 0));
+
     // Config tab bar with extra label padding and uniform, centered tab widths.
     // Wrap items in BeginPaddedTabItem between BeginPaddedTabBar/EndTabBar.
     bool BeginPaddedTabBar(const char* str_id, ImGuiTabBarFlags flags = 0);