Change app ID to com.outlay.app, add AppStream metadata, fix toast visibility

outlay-gtk/data/com.outlay.app.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Name=Outlay
+Comment=Personal income and expense tracker with budgets, charts, and multi-currency support
+Exec=outlay-gtk
+Icon=com.outlay.app
+Terminal=false
+Type=Application
+Categories=Office;Finance;GTK;
+Keywords=expense;budget;money;finance;income;tracker;currency;receipt;
+StartupNotify=true
+SingleMainWindow=true