(feature) Allow to use chili-widgets when engine's menu is disabled, too.

[knorker]

It could be useful if chili-widget still worked even when the engine's GUI is hidden.
(player pressed F5 or textcommand "hideinterface 1")
Chili has Hide/Show functions so that a widget can decide on its own if/what to hide or show in that situation.
At the moment that decision seems to be taken away by the chili framework. *

A usecase might be widgets that are supposed to be visible even with "all" GUI disabled:
For example a "return to normal mode" button that only shows when the GUI is hidden.
(allows players to get their GUI when pressing F5 was maybe on accident.)

Or maybe widgets that go a bit into "debug territory" like widget selector might wish to ignore Spring.IsGUIHidden() too.

Even more radically:
In some situations none of the engine'S GUI is needed but chili-widgets should show:
Example 1: menus with animated gameplay in background.
Example 2: Mod that does all its GUI in Lua and has no more need for any of the engineGUI, even during normal play.

Being able to use "hideinterface 1" would be more elegant than fumbling to disable all the elements seperately. (and maybe restore them too..)

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*Not 100% sure on that, but I do not see where such option could be hidden for example here:

function widget:MousePress(x,y,button)
if Spring.IsGUIHidden() then return false end