Bunch of additional refinements to the iemgui numbox

* Ability to edit incrementally with text and mouse (e.g. backspace erases only one digit)

* Better use of hte '>' as a cursor

* Key presses start a new number from scratch unless user has dragged a number and established a new number by dragging, or unless they shift+clicked when they gained focus on the numbox, in which case numbox will allow for user to append to the existing value

* Shift + backspace erases everything, while backspace erases only one character

* When there are no digits entered, the value reverts to 0

* Pressing return/enter removes the trailing '>' until a key is once again pressed

* Changing values with mouse also removes the trailing '>'

* Resizing across only x and y axes now works (vertical resize still inevitably affects the width due to change in character length, but it now never exceeds the x value it started with during that particular click'n'drag action)