feat(launcher): add configurable terminal command

The terminal command for launching applications was previously hardcoded to 'kitty', causing issues for users without it installed.

This change introduces a new setting, 'appLauncher.terminalCommand', allowing users to specify their preferred terminal emulator. The default value is set to 'xterm -e'.

The implementation includes:
- Defining the setting in 'Commons/Settings.qml'.
- Adding a text input in the launcher settings tab.
- Updating the application plugin to use the new setting.

Assets/settings-default.json

@@ -109,7 +109,8 @@
     "backgroundOpacity": 1,
     "pinnedExecs": [],
     "useApp2Unit": false,
-    "sortByMostUsed": true
+    "sortByMostUsed": true,
+    "terminalCommand": "xterm -e"
   },
   "dock": {
     "autoHide": false,