Summary: 9c5062147a introduced the `getTextSize()` to all GUIObjects for
getting the rendered size of a text. 8190293f8b made it only available
to CText. `getTextSize()` can also be useful for CButtons so this diff
adds the function back to CButtons. There seem to be no other GUIObjects
besiders CText and CButton where `getTextSize()` makes sense.
Reviewed By: wraitii
Differential Revision: https://code.wildfiregames.com/D3195
This was SVN commit r24378.
