Add a read-only IsMinimised property to Window

Objo desktop apps can minimise a window through the platform window controls or by calling Window.Minimise(), but app code currently has no way to ask whether a window is minimised.

Window.Visible does not answer that question: it reports whether the window is shown or hidden, and it correctly stays True while a window is minimised (the same behaviour as Xojo, where a minimised window is still visible). This leaves a gap next to the existing read-only IsMaximised and IsFullScreen properties.

Add a read-only IsMinimised Boolean property to Window that reports True only while the window is in the minimised state, so apps can detect and respond to minimisation (for example from the Resize event handler).