Tweaked UI layout:
 - The controls now resize with the window.
 - Added padding.
 - Added a minimum size to the window to prevent distortions when users reize the window very small.
Renamed WidthAndHeightControl to ImageScaleControl.  Also removed extraneous frame in InitImageScaleControl.
Switched to using ttk widgets instead of tk ones.  Also fixed a bug wheree keyword args were not being properly forwarded in the widget initializers.
Added a checkbox control for the aspect ratio with widget helper CheckboxControl, which wraps ttk.Checkbutton.  Also changes super-class __init__ calls to use super() instead of explicitly specifying the superclass.  Also tweaked some padding.
Fixed LabelledEntryControl.SetText using ttk.END instead of tk.END.  Also changed ImageScaleControl.SetWidthAndHeight to not return any value.
Changed ImageScaleControl's Set{Width,Height} methods to scale appropriately if GetKeepAspectRatio() is true.
Added enabling/disabling the sizing control and have the sizing control toggle the enabled-ness of the height field when the maintain-aspect-ratio checkbox is toggled.
Renamed IntegerEntry::_IsValidStatid and made it a member method.
Fixed a bug in IntegerEntry.__init__ where keywordArgs wasn't being properly forwarded to super.__init__.
Updated ImageScaleControl to have a default aspect ratio (1) and changed GetWidth/GetHeight's callers to do validation instead of GetWidth/GetHeight themselves, since the knowledge of how to handle an error is known only at the higher level.
Created Label and Entry widgets, which add convenience methods to ttk.Label and ttk.Entry respectively.
Split _IntegerEntry._IsTextValid(self, text) into _IsTextValid(self, oldText, newText) and _IsTextValidStatic(text).
Moved the validation registration from IntegerEntry to Entry.
Bound Control-a to select all for the Entry widget.
Switched the Entry widget to use a textvariable to set the value.
Allow the LabelledEntryControl widget to pass keyword-args to its label and entry (previously it only supported passing keyword-args to the enclosing frame).
Fixed LabelledEntryControl still using the removed enable/disable-entry helpers.
Changed ImageScaleControl's width and height entries to clear themselves when unable to scale, instead of doing nothing.
When ImageScaleControl's keep-fixed-aspect-ratio-checkbox is checked, update the height when the width is changed. Fixes #19.