UnityMenuSystem

Converted forked project from uGUI to UITK.

It's possible to go back to uGUI

uGUI components like Canvas, Image and Text are still in the prefabs, just disabled. The logic is in the source code too, just commented out. So it's possible to go back to uGUI with just a little work - enabling uGUI components, disabling UITK's UIDocument, commenting UITK code and uncommenting uGUI code.

Changes

• Added UIDocument to each prefab
• Added UXML for each Menu, and a shared USS
• Added PanelSettings
• Added RuntimeTheme.tss with some overrides in default-override.uss so that buttons would look similar to uGUI buttons
• Each Menu now has UIDocument field, from its rootVisualElement we query UXML's elements and add logic in Awake
• As UIDocument clears content OnDisable, instead of deactivating GOs, their rootVisualElements are now just hidden with DisplayStyle.None