This pull request will allow you to add math to text boxes in the keyframe and marker GUIs. This allows you to just add -20 or +10 to the end of value and have the value change by that amount.

Changes

• Make the textboxes bigger to give you more space to write expressions
• Allow negative numbers in the time field
• Add EvalEx Dependency to do the math parsing
• Add GuiExpressionTextField which takes care of parsing and checks if the expression is valid (This class probably needs to be moved to a better place, but I'm not sure where to put it. Would this belong in jGui?)
• Add checks everywhere to make sure the user can't submit an invalid expression
• Have the textbox turn red if the expression is invalid