NUI: Add new specialised KeyActivatedButton and UIWarnButton widgets to mimic existing UI behaviour

[BenjaminAmos]

Description

This pull request adds the KeyActivatedButton and UIWarnButton widgets to mimic the UI behaviour of the pre-existing SolUiControl buttons.

• A KeyActivatedButton widget will activate when a key is pressed. If the key is held, then it will act the same as if a left-mouse button was holding it down.
• A UIWarnButton can flash an orange colour to notify the user that an event has occured, such as when new items are present in the inventory. It is also used in the tutorial to indicate which buttons to click on.

The pull request also adds a separate button disabled texture (previously missing) and moves the MigTestScreen into a seperate test screens package.

Testing

• Start a new game, or continue an existing one
• Open the console with the ` key or F1
• Use the command showNUIScreen engine:solButtonsTestScreen
• Press the keys shown on the buttons to press the corresponding buttons
• Hold the keys shown on the buttons to hold the corresponding buttons down
• The I button should be flashing orange

Notes

• This pull request depends on MovingBlocks/TeraNUI#63 for testing.
• These changes were included in #588 but are being submitted separately so that work can start on porting other UI screens to NUI

[Cervator]

With #588 merged this is partially obsolete, but the additional test screen is new and the old one moved. @BenjaminAmos want to redo that setup real quick or apply anything else left from this PR? No rush of course :-)

[BenjaminAmos]

The new test screens should probably go in a separate module and all the other changes in this PR merged as part of #588, so I'm going to close this for now.