GPII-1014: Replace the slider adjuster for cursor size with plus/minus text field adjuster

The bug report: http://issues.gpii.net/browse/GPII-1014

This pull request also upgraded the infusion library to be in sync with the latest master: https://github.com/fluid-project/infusion/
The German for "plus" is indeed "plus" - http://dict.leo.org/ende/index_en.html#/search=plus - however, it seems to me that we should pick a more meaningful word across all languages to refer to this function, rather than to just describe the icon. Perhaps the words should in fact be "larger" and "smaller" rather than "plus" and "minus".