refactor: normalize ShowAddButton to IsAddButtonVisible

[sundaramramaswamy]

Rename ShowAddButton() → IsAddButtonVisible() to align with WinUI's Is* naming convention for boolean properties (Control.IsTabStop, TabView.IsAddTabButtonVisible, etc.).

• IsTabStop — kept as-is (already matches WinUI naming per review feedback)
• ShowAddButton → IsAddButtonVisible (imperative verb → Is* boolean)
• Old ShowAddButton kept as [Obsolete] shim for backward compatibility
• reactor.api.txt updated in both locations

Fixes #268

[codemonkeychris]

I think we should align with the WinUI names, IsXXX

[sundaramramaswamy]

@codemonkeychris Addressed — updated to align with WinUI's Is* naming convention:

• IsTabStop stays as-is (already matches Control.IsTabStop)
• ShowAddButton → IsAddButtonVisible (matches TabView.IsAddTabButtonVisible pattern)

Force-pushed with the narrower scope.

[sundaramramaswamy]

Thanks for the direction, Chris. Agreed on ShowAddButton — that's clearly an imperative outlier and is now IsAddButtonVisible.

For the remaining adjective modifiers (Disabled, Editable, Selectable, Closable, Active), wanted to surface a trade-off before expanding scope:

The adjective forms are sugar over the Is* record properties — e.g. Closable(false) sets IsClosable = false, Editable() sets IsEditable = true. Renaming these to IsClosable(), IsEditable(), IsActive() is straightforward.

The one case that doesn't map cleanly is Disabled(). The WinUI property is IsEnabled — opposite polarity. Today you write Button("Save").Disabled(), which reads naturally. The Is* equivalent would be IsEnabled(false), which loses the ergonomic default.

Should we do (1) or (2)?

1. rename all five to Is* (with IsEnabled(false) replacing Disabled())
2. rename the four that map directly (Editable → IsEditable, Closable → IsClosable, Active → IsActive, Selectable → IsTextSelectionEnabled) and keep Disabled as-is since it inverts IsEnabled?

[codemonkeychris]

The one case that doesn't map cleanly is Disabled(). The WinUI property is IsEnabled — opposite polarity. Today you write Button("Save").Disabled(), which reads naturally. The Is* equivalent would be IsEnabled(false), which loses the ergonomic default.

Please look at the equivalent in WinUI and follow the pattern to align.

[sundaramramaswamy]

@codemonkeychris Addressed your feedback — all boolean modifiers now use exact WinUI property names (IsEnabled, IsReadOnly, IsEditable, IsClosable, IsActive, IsVisible, IsTextSelectionEnabled, IsDisabledFocusable, IsLightDismissEnabled, IsAddTabButtonVisible). Old names kept as [Obsolete] shims. Ready for re-review when you get a chance.