gdesktopappinfo: Take in account the desktop Path to find executables

Desktop files can provide the executable working path and that can be
used to pick the file to launch.

So take it in account.

gio/tests/appinfo-test-path.desktop.in

@@ -0,0 +1,22 @@
+[Desktop Entry]
+Type=Application
+GenericName=generic-appinfo-test-path
+Name=appinfo-test-path
+Name[de]=appinfo-test-de
+X-GNOME-FullName=example
+X-GNOME-FullName[de]=Beispiel
+Comment=GAppInfo example
+Comment[de]=GAppInfo Beispiel
+Path=@installed_tests_dir@
+TryExec=apps
+Exec=appinfo-test --option %U %i --name %c --filename %k %m %%
+Icon=testicon.svg
+Terminal=false
+StartupNotify=true
+StartupWMClass=appinfo-path-class
+MimeType=image/png;image/jpeg;
+Keywords=keyword1;test keyword;
+Categories=GNOME;GTK;
+X-JunkFood=Burger
+X-JunkFood[de]=Bratwurst
+X-JunkFood[it]=