[ResultDisplay] Require QPlaintextEdit since Qt 4.6 is required
[Editor] Paint a customized cursor according to a color scheme
Handle zoom and scroll events from within Editor instead of MainWindow

New shortcuts:
   - Scroll up one page:      PageUp
   - Scroll down one page:    Page Down
   - Scroll up one result:    Shift + PageUp
   - Scroll down one result:  Shift + PageDown
   - Increase font size:      Shift + Up
   - Decrease font size:      Shift + Down

The result display does not get focus.
Style all ResultDisplay and Editor visual elements via color scheme