<p>
An entry is a one line text editor. It is typically used to obtain textual input from user,
such as a file name. 
<p>
To enter text into an entry the user moves the pointer into the entry, causing the
entry to take focus and making a red cursor to appear. The user can then enter characters
and they will be inserted at the point of the cursor. The cursor can be moved by clicking
mouse button 1 anywhere in the entry's text. Text in an entry can be selected by pressing and
dragging mouse button 1. Selections can be pasted into an entry by pressing mouse button 2
in the entry. The entry widget supports a few emacs like editing commands.
<p>
When a return key or the tab key is pressed in an entry, and if it has callbacks, 
the callbacks are invoked.
