feat: add tray-only taskbar option

[nbali]

This pull request adds a new feature allowing users to hide the timer window from the taskbar when it is shown in the notification area. The implementation includes a new setting, command-line option, and context menu item, as well as the necessary logic to update the window's visibility in the taskbar based on user preferences.

New feature: Hide timer from taskbar when in notification area

• Added a new user setting HideFromTaskbarWhenInNotificationArea to application settings and configuration files. [1] [2] [3]
• Implemented a new command-line option --hide-from-taskbar-when-in-notification-area (aliases: -hf, /hf) to control this behavior, including parsing, usage documentation, and default handling. [1] [2] [3] [4] [5] [6]
• Updated the context menu with a new checkable item to toggle hiding the timer from the taskbar when shown in the notification area, including resource strings and UI logic. [1] [2] [3] [4] [5] [6]
• Modified TimerWindow logic to update the ShowInTaskbar property and related UI when the new setting changes, and ensured event handlers are properly managed. [1] [2] [3] [4]
• Updated documentation in FAQ.md to describe the new feature and its command-line usage.

These changes enhance user control over the application's window visibility and provide both UI and command-line access to the new behavior.

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Command-line support

• Added HideFromTaskbarWhenInNotificationArea property to CommandLineArguments, following the existing ShowInNotificationArea pattern (most-recent-options and factory-defaults resolution).
• Added --hide-from-taskbar-when-in-notification-area switch (aliases -hf, /hf) with on|off|last semantics.
• Wired the parsed value into AppEntry.SetGlobalSettingsFromArguments to update Settings.Default.HideFromTaskbarWhenInNotificationArea.

Documentation

• Documented the new switch in Usage.txt, including its entry in the factory-defaults list.
• Added a mention of the setting and its command-line equivalent in FAQ.md, alongside the existing notification-area minimizing instructions.

hourglass -n on -hf on 5

starts a 5-minute timer that shows its notification-area icon and hides the timer window from the taskbar while minimized.