mouse and toolbar improvements

[andrejv]

Converted from SourceForge issue 2814129, submitted by dougedmunds

These are on my wish-list:

• mouse: add right-click functionality when on a 'horizontal line' for pasting (after using mouse to select all and copy some other entry)
• toolbar: add a "select all" button and a "paste" button to the toolbar
• mouse: add single-click combination for "select all then copy" to right click context menu
• mouse: double-click that selects the entire input. Suggested locations: double clicking on the system prompt ">>" ; the space just to the right of >>; after the last input character. This would also work on the (%i##) if the expression has been evaluated, but do not include the input number (%i##) part of the line.
• mouse: similar to the preceding item, but for output after expression evaluated. Double-click that selects entire output, without the output number (%o###) part of the line
• toolbar: a 'see input as xml' button. This would show the ascii input as xml, but without doing any calculations. The effect is similar to doing this sequence of steps: 1. simp:false; 2. ; 3. ; 4. simp:true.
  However, note that depending on the packages loaded, I find that simp:false cannot always be set.