This makes it more consistent with the other settings windows
- Use editable cell for entry
- Use pane to resize both treeviews
- Make preview fixed size and on bottom
- Enable search for event name
Xtext's transparency barely worked on windows, didn't work on any modern
linux wm and used fake transparency.
This uses gtk's built in window opacity that works on more systems and
is real transparency.
Text area only transparency may return with a transition to cairo, if it
works on Windows.
