Replace GimpSizeEntry widget

by Enrico Schröder for GNU Image Manipulation Program

The GimpSizeEntry widget in its current form, which is used across the user interface for entering values and their corresponding units (e.g. image dimensions, lengths, etc.), has a couple of issues both from a users and a developers standpoint. The project is to rewrite the widget being based on GtkEntry with the ability to directly enter and display the desired unit in the text field itself instead of in an external control. Simple mathematical terms for the input of values in different units will be possible using the existing parser (gimpeevl).