Add per-item conditional property to gate generated code

[Randalphwa]

Motivation

Platform checks in wxUiEditor only covered Windows/Unix/Mac. This PR allows the dev to also use a conditional in addition to the platform checks. E.g., you can now add something like 'MY_FEATURE_ENABLED' and the to prevent compiling or using the generated code unless condition is true (in addition to the selected platforms being true).

Changes

• New conditional string prop (prop_conditional) on widgets/windows. Empty by default; when set it gets AND-ed into the platform check during code generation. Defined in interfaces.xml and registered in the enum/help maps so it round-trips and is undoable.
• Code generation: construction, event bindings, member variables, and the public/protected header sections all pass the conditional through. When an item targets all platforms, it now emits a plain conditional block instead of a platform test.
• Each language strategy (C++, Python, Ruby, Go, TypeScript, Julia, LuaJIT, Fortran, FFI) spells the condition in its own syntax through a new EmitConditionalOnly.
• Property grid: the platforms row gets an indented string child for the conditional, via a small ConditionalFlagsProperty subclass.
• XRC: logs a warning and ignores the property, since XRC cannot express it.

Closes #1557