applets/digital-clock: Add middle-click to open calendar app

Original commit message(s)

• feat(digital-clock): add middle-click to open calendar app
• fix(digital-clock): use onActivated for calendar app selection

Reason for the change

Currently, users can only view the calendar popup by left-clicking on the digital clock widget. Users who prefer external calendar applications (like Merkuro, KOrganizer or Thunderbird) for full-featured calendar management have no quick way to launch them from the clock widget. This MR adds configurable middle-click support to open a calendar application directly from the digital clock plasmoid, while preserving the existing left-click behavior (opening the calendar popup). Users can:

• Toggle the middle-click calendar feature on/off via configuration
• Choose their preferred calendar application from a list of installed calendar apps (auto-detected via text/calendar MIME type)

Test plan

1. Right click in the digital clock → Configure Digital Clock → Calendar
2. Verify a new "Middle click" section appears with "Open calendar application" checkbox
3. Enable the checkbox and verify a dropdown is enabled with installed calendar applications
4. Select a calendar application from the dropdown
5. Middle-click on the digital clock widget and verify the selected calendar application launches
6. Verify left-click still opens/closes the calendar popup as before
7. Disable the checkbox and verify middle-click does nothing (or applies default behavior)

Screenshots or screen recordings

Before	After

Bugs fixed

N/A