Help and uiobject.message updates to support i8n

[brickbots]

To allow more flexible and thorough translation of the UI these changes are proposed:

Switch help system from using pre-rendered images to text
Move all help text to strings that can be translated while maintining the ability to show button images, split pages, and include arbitrary images

• Work out specific token that represent buttons and would be replaced with images on rendering
• Decide on a token to indicate a page break
• Implement system to include images as needed

Allow multi-line messages
To provide more flexibility in translated strings, allow a newline char to be used to break text in .message method

[jscheidtmann]

mrosseel is working on text help in #326