Fix AppImage binary path resolution

When running inside an AppImage, the bundled ld-linux wrapper causes
/proc/self/exe to resolve to the dynamic linker rather than the actual
binary. Both kiway.cpp and paths.cpp now check the APPDIR environment
variable and construct the correct executable path, falling back to the
standard path resolution when not in an AppImage.

common/paths.cpp

@@ -668,18 +668,35 @@ const wxString& PATHS::GetExecutablePath()
         // Use bundle root helper which handles symlink resolution and aux binaries
         exe_path = getOSXBundleRoot() + wxT( "/" );
 #else
-        wxString bin_dir = wxStandardPaths::Get().GetExecutablePath();
+        wxString envPath;
 
-        // Use unix notation for paths. I am not sure this is a good idea,
-        // but it simplifies compatibility between Windows and Unices.
-        // However it is a potential problem in path handling under Windows.
-        bin_dir.Replace( WIN_STRING_DIR_SEP, UNIX_STRING_DIR_SEP );
+        // When running inside an AppImage, the bundled ld-linux is invoked as a wrapper
+        // which causes /proc/self/exe to resolve to the dynamic linker rather than the
+        // actual binary. Use APPDIR to construct the correct executable path.
+        if( wxGetEnv( wxT( "APPDIR" ), &envPath ) )
+        {
+            envPath.Replace( WIN_STRING_DIR_SEP, UNIX_STRING_DIR_SEP );
 
-        // Remove file name form command line:
-        while( bin_dir.Last() != '/' && !bin_dir.IsEmpty() )
-            bin_dir.RemoveLast();
+            if( !envPath.EndsWith( wxT( "/" ) ) )
+                envPath += wxT( "/" );
 
-        exe_path = bin_dir;
+            exe_path = envPath + wxT( "usr/bin/" );
+        }
+        else
+        {
+            wxString bin_dir = wxStandardPaths::Get().GetExecutablePath();
+
+            // Use unix notation for paths. I am not sure this is a good idea,
+            // but it simplifies compatibility between Windows and Unices.
+            // However it is a potential problem in path handling under Windows.
+            bin_dir.Replace( WIN_STRING_DIR_SEP, UNIX_STRING_DIR_SEP );
+
+            // Remove file name form command line:
+            while( bin_dir.Last() != '/' && !bin_dir.IsEmpty() )
+                bin_dir.RemoveLast();
+
+            exe_path = bin_dir;
+        }
 #endif
     }