Introduce a binary maximized mode, to help the player from minimizing

There's two bugs I noticed on desktop (because the window will resize):

1. The player will fail to show properly when maximized (this bug doesn't happen when resized). This makes it unusable and you need to touch the tiny bar at the bottom to fix this.
2. When resizing, the player will sometimes disappear back into minibar form.

To solve the first issue, we're now doing the resize check on the onHeightChanged signal of the ApplicationWindow itself which seems to be more reliable.

To prevent the player from erroneously minimizing on resize (because we check it's new contentHeight, which is invalid) use a binary flag to record whether the player has been opened. This used to be a readOnly property, which was unused(?)