Preview panel 2

Fix issues from https://trello.com/c/8wA2ry0T/65-ui-fixes-rendering-controls-preview

To test:
 - Preview panel buttons Undo/Redo etc have text. Check appearance, disabled/enabled OK etc.
 - Label updated "User Settings"
 - Channel button names only truncated after 7 characters.
 - Save button more clearly indicates when disabled.
 - Paste button is only enabled if compatible rendering settings are copied. Can still paste settings from the same image they were copied from. E.g. copy from an image with 3 channels, go to 2 channel image (Paste disabled) then 'Copy' and the 'Paste' will become enabled.