[2.3] [Re-build] Prevent combobox lists being taller than the screen, mainly from windows

Fixing bug introduced by me *shame*, the dropdown list of a combobox (when containing many items, 50 in the example) can overflow the screen and hide the pagination bar, see the following screencap:

![modx combobox-listheight](https://cloud.githubusercontent.com/assets/445501/4475038/e3bee450-4967-11e4-9081-3bd5a2837c00.gif)

the bug is coming from a forced height: 100% !important; in _forms.scss, this was somewhen done in the 2.3 UI re-styling process to override the calculated height from extjs, reverted this change and removed that rule...