MSIX: implement initial version of notifications library (#1178)

doc/devdocs/common.md

@@ -43,3 +43,67 @@ Helper methods for the settings.
 
 #### Start visible helper: [header](/src/common/start_visible.h) [source](/src/common/start_visible.cpp)
 Contains function to test if the Start menu is visible.
+
+# Toast Notifications
+
+#### Notifications API [header](/src/common/notifications.h) [source](/src/common/notifications.cpp)
+To use UWP-style toast notifications, simply include the header and call one of these functions:
+
+```cpp
+    void show_toast(std::wstring_view message);       // #1
+    
+    void show_toast_background_activated(             // #2
+      std::wstring_view message,
+      std::wstring_view background_handler_id,
+      std::vector<std::wstring_view> button_labels);
+```
+We might add more functions in the future if the need arises, e.g. `show_toast_xml` which will accept raw XML for rich customization.
+
+Description:
+- `#1` is for sending simple notifications without any callbacks or buttons
+- `#2` is capable of showing a toast with multiple buttons and background activation
+- `message` is a plain-text argument
+
+Implement a toast activation handler/callback as a function in [handler_functions.cpp](/src/common/notifications_winrt/handler_functions.cpp) and register its `background_handler_id` via `handlers_map`, e.g.:
+
+```cpp
+// Your .cpp where you'd like to show a toast
+
+#include <common/notifications.h>
+
+void some_func() {
+// ...
+  notifications::show_toast_background_activated(
+    L"Toast message!",                                                  // text displayed in a toast
+    L"awesome_toast",                                                   // activation handler id
+    {L"Press me!", L"Also could press me!", L"I'm here to be pressed!"} // buttons in a toast
+  );
+```
+
+```cpp
+// handler_functions.cpp
+void awesome_toast_handler(IBackgroundTaskInstance, const size_t button_id)
+{
+  switch(button_id)
+  {
+    case 0:
+      // handle "Press me!" button click
+    case 1:
+      // handle "Also could press me!" button click
+    case 2:
+      // handle "I'm here to be pressed!" button click
+  }
+}
+
+namespace
+{
+  const std::unordered_map<std::wstring_view, handler_function_t> handlers_map = {
+    // ...other handlers...
+    {L"awesome_toast", awesome_toast_handler}
+  };}
+
+```
+
+Note: since _background activation_ implies that your toast handler will be invoked in a separate process, you can't share data directly from within a handler and your PT process. Also, since PT is currently a Desktop Bridge app, _foreground activation_ is [handled the same as background](https://docs.microsoft.com/en-US/windows/uwp/design/shell/tiles-and-notifications/send-local-toast-desktop-cpp-wrl#foreground-vs-background-activation), therefore we don't make a dedicated API for it. You can read more on the rationale of the current design [here](https://github.com/microsoft/PowerToys/pull/1178#issue-368768337).
+
+