Disable widgets

[xarvic]

An implementation of a disabled state for widgets (issue: #746 ).
It adds the following:

• set_enabled(bool) for EventCtx and UpdateCtx which changes if the state of the current widget is "manually disabled".
• is_enabled() -> bool for all Context which returns false if this widget or any of its ancestors is manually disabled".
• LifeCycle::EnabledChanged(bool) which is called every time when is_enabled() would return a different value than before.

The behaviour of focusing widgets was changed. The WidgetPod now has field can_auto_focus that is used to determine if this widget should get focused as a result to focus_pref or focus_next. If the widget is focused when it gets disabled, it resigns the focus and is`nt able to request the focus until it is enabled again.

While disabled, a widget recieves only some of the event. MouseMove, MouseDown, MouseUp, Zoom and MouseWheel are blocked. But i`m not sure if this is desirable, maybe a scroll widget should still be able to scroll its content.

Problems:

• widgets are always enabled until the first call to update or event, therefore for the moment using EnabledIf is not a reliable safety mechanism to prevent the user from doing illegal operations.

Todo:

• make widget inactive when disabled

[xarvic]

This was way to complicated and didn't work as well as expected. I'm closing this Issue for now.

[cmyr]

@xarvic this is a definitely a harder problem than it might seem like at first, happy to talk about it sometime if you'd like.