musrview, musrFT, musrt0: show their own window/dock icon instead of the generic ROOT one.

The ROOT canvas sets the macro_s.xpm icon and WM_CLASS "ROOT". Set a compiled-in
xpm icon and a per-tool class hint, and install matching .desktop files and icons.
Since desktops only search XDG data dirs, .desktop files and icons (incl. musredit)
are now installed to MUSRFIT_XDG_DATADIR (default: prefix share for /usr and
/usr/local, ~/.local/share for prefixes under $HOME, /usr/local/share otherwise).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/classes/PFourierCanvas.cpp

@@ -39,6 +39,8 @@
 
 #include "PFourierCanvas.h"
 
+#include "icons/musrFT_icon.xpm"
+
 #define YINFO  0.2
 #define YTITLE 0.95
 
@@ -1119,6 +1121,9 @@ void PFourierCanvas::InitFourierCanvas(const Char_t* title, Int_t wtopx, Int_t w
   // add canvas menu if not in batch mode
   if (!fBatchMode) {
     fImp = static_cast<TRootCanvas*>(fMainCanvas->GetCanvasImp());
+    // replace the generic ROOT window icon; the class hint lets the desktop match musrFT.desktop
+    fImp->SetIconPixmap(const_cast<char**>(musrFT_icon_xpm));
+    fImp->SetClassHints("musrFT", "musrFT");
     fBar = fImp->GetMenuBar();
     fPopupMain = fBar->AddPopup("MusrFT");