add keyboard shortcut to show the current position in seconds

Very helpful for setting up the fade out position.

README.md

@@ -45,6 +45,7 @@ The following other controls are available:
 | Mouse Wheel | manually scroll through the metadata bar (stops autoscrolling)
 | **A** | stop / resume autoscrolling
 | **F** | slowly fade out the song
+| **P** | show the current track position in seconds (hold key to update continuously)
 | **V** | show the TrackMeister and libopenmpt version numbers
 | **F5** | reload the current module and the application's configuration
 | **F11** | toggle fullscreen mode
@@ -154,6 +155,7 @@ Options can also be specified on the command line, in the syntax "`+key=value`"
 - Listen to the tracks again and set track-specific options in `tm.ini`. For each track you want to set special options for, you create a new section based on the running order, e.g. `[08*]` for the options of the file in the aforementioned example. <br> Typical per-track options are:
   - If the track shall loop, set `loop = true`. If you also set `fade out after loop = true`, the track will also fade out at this point.
   - If the track shall fade out at some position, set `fade out at =` to the number of seconds after which fading shall start (e.g. `fade out at = 123` fades out after 123 seconds, i.e. 2 minutes and 3 seconds).
+    - To determine the proper value, just pause playback at the position where you want the fade to start (using the **Space** key) and press the **P** key to display the position. The raw value in seconds can be directly used for the `fade out at` option.
   - If the track shall be played in mono, set `stereo separation = 0`.
   - If the track shall be played with some volume ramping, set `volume ramping = 10`; if it shall not use ramping, set `volume ramping = 0`.
   - You can also override the track's title/artist metadata by setting it manually, e.g. `title = Professional Tracker` and `artist = H0ffman feat. Daytripper`.

src/app.cpp

@@ -256,6 +256,9 @@ void Application::handleKey(int key, bool ctrl, bool shift, bool alt) {
         case 'V':  // show version
             toastVersion();
             break;
+        case 'P':  // show position
+            toastPosition();
+            break;
         case 'F':  // start fade-out
             fadeOut();
             break;
@@ -385,6 +388,15 @@ void Application::toastVersion() {
     toast(ver);
 }
 
+void Application::toastPosition() {
+    if (!m_mod) { return; }
+    char line[80];
+    AudioMutexGuard mtx_(m_sys);
+    auto sec = m_mod->get_position_seconds();
+    sprintf(line, "current track position: %d:%02d (%.2f seconds)", int(sec) / 60, int(sec) % 60, sec);
+    toast(line);
+}
+
 void Application::updateImage(ExternalImage& img, const std::string& path, int channels, const char* what) {
     int64_t mtime = PathUtil::getFileMTime(path.c_str());
     if ((path != img.path) || (mtime > img.mtime)) {

src/app.h

@@ -167,6 +167,7 @@ class Application {
     void toast(const char* msg);
     inline void toast(const std::string& msg) { toast(msg.c_str()); }
     void toastVersion();
+    void toastPosition();
     void fadeOut();
     void startScan(const char* specificFile=nullptr);
     void runScan();