Fixed some bugs in the ui

- Removed the option for inline completion, because the new interface
only has popup completion.
- Changed the cursor to an IBeamCursor for editable TextItems
- Pressing Shift+Del repeatedly does not remove the last entry anymore,
it should not be possible to get a worksheet without any entry.
- Entries that are wider than the window now make the horizontal
scrollbar appear, and the scroll width is adjusted appropriately
when new items are created or existing items are destroyed.