Panel Settings Dialog (again)

[Jasper Huijsmans]

Hi everyone,

I have just committed another attempt at implementing the user interface 
for customizing the panel. Please test it and let me know what you think.

I have split up the functionality again and added a few options to the 
right-click menu. I believe this makes things much easier to discover, 
maybe at the expense of making certain things take more time. However, 
customizing the panel is not something most users will do very often (at 
least, according to Erik's survey).

The plugin right-click menu now becomes:

  Properties
  Move
  ---
  Remove
  ---
  Add New Item
  Customize Panel

I feel this is a reasonable compromise.

TODO:
- add item next to the plugin that was clicked on to get the menu
- show properties dialog for newly added items
- maybe put panel properties on 1 page again, instead of using two tabs

Please tell me what you think.

Thanks,
	Jasper