Fix Script Editor default window size

loadPreferences() now uses default window width and heights if getSize()
returns 0 for either dimension. Fixes bug in Ubuntu 12.04 where Script
Editor is initialized with 0 pixel window.

Signed-off-by: Curtis Rueden
Merge branch 'script-editor-default-window-size'

This is an adapted version of Sam Inverso's patch from Fiji PR #53:
    https://github.com/fiji/fiji/pull/53