fix(input/#2428) - Input text component + textInputFocus context key …

…fixes (#2433)

This is more groundwork being laid to support #528 , which requires us to refine our focus management story. This enables features to conditional expose context keys (which may be from nested items - like the sidebar exposing `textInputFocus` when an input control has focus in a sidebar item...)

This moves `InputText` to a component - a rich component that exposes context keys, potentially keybindings, subscriptions, etc. I'll follow this same model for some additional rich components I'll be adding to support #528 

I started a debug view for input / context keys in #2427 , it's ugly right now - but it shows the `textInputFocus` key being set as expected in various scenarios:

![2020-09-10 15 32 01](https://user-images.githubusercontent.com/13532591/92817678-aef46280-f37b-11ea-82d7-fb5c4046c203.gif)

This fixes the `textInputFocus` part of #2428 , and requires a revery fix for drop-shadow to give more emphasis around text-inputs having focus: revery-ui/revery#999