Add a -T option that sets window title.

`cool-retro-term -T myretrosession` sets window name to 'myretrosession'

This is useful especially for people who rely on wmctrl to change windows, now I
can do `wmctrl -a myretrocession` to focus on cool-retro-term.

-T was chosen since it matches classic xterm(1) and other terminal emulators

app/qml/main.qml

@@ -62,8 +62,10 @@ ApplicationWindow{
         __contentItem.visible: mainMenu.visible
     }
 
+    property string wintitle: appSettings.wintitle
+
     color: "#00000000"
-    title: terminalContainer.title || qsTr("cool-retro-term")
+    title: terminalContainer.title || qsTr(appSettings.wintitle)
 
     Action {
         id: showMenubarAction