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.

Commons/Settings.qml

@@ -225,6 +225,7 @@ Singleton {
       property list<string> pinnedExecs: []
       property bool useApp2Unit: false
       property bool sortByMostUsed: true
+      property string terminalCommand: "xterm -e"
     }
 
     // dock