feat: add "Hide window title bar" option independent of window title content

[nbali]

This pull request adds a new feature that allows users to hide the timer window title bar via both the command line and the context menu. It introduces a HideWindowTitleBar option throughout the codebase, updates the UI and serialization logic to support this setting, and documents the new option in the usage and help files.

New Feature: Hide Window Title Bar

• Added a HideWindowTitleBar option to TimerOptions, CommandLineArguments, and TimerOptionsInfo, including support for serialization, deserialization, and property change notifications. [1] [2] [3] [4] [5] [6] [7]
• Implemented command-line parsing for the new --hide-window-title-bar option, with support for aliases and default values. [1] [2] [3] [4]
• Updated the context menu to include a "Hide window title bar" item, with proper localization, state synchronization, and event handling. [1] [2] [3] [4] [5] [6]
• Modified the window logic and XAML to use the new IsWindowTitleBarHidden property, ensuring the title bar is hidden when either the old or new option is set. [1] [2] [3] [4]
• Documented the new option in README.md and updated Usage.txt to reflect the new command-line flag and its description. [1] [2] [3]

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Setting "Window title" to "None" was the only way to hide the timer window's title bar, but that also removed the title from the taskbar. There was no way to show a meaningful title in the taskbar while hiding the title bar on the window itself.

Option plumbing

• Added HideWindowTitleBar bool to TimerOptions, wired into construction, copy/clone, and TimerOptionsInfo serialization.

Window behavior

• Added computed IsWindowTitleBarHidden on TimerWindow (WindowTitleMode == None || HideWindowTitleBar), notified whenever either source option changes.
• TimerWindow.xaml now drives the title bar visibility from this single property instead of duplicating the WindowStyle/WindowChrome setters per condition.

UI / CLI

• New "Hide window title bar" checkable item in the timer window context menu, alongside the existing "Window title" submenu.
• New --hide-window-title-bar command-line switch (aliases -hb, /hb), documented in Usage.txt and README.md.

Now a "Timer title" (or any other) window title mode can be combined with "Hide window title bar" to show a meaningful title in the taskbar without the title bar cluttering the window itself.