Fix window height bug in Zotero preferences, and reorganize code

Prefpanes are now in separate overlays, which fixes the age-old resizing
bug when switching between panes (at least on OS X) that varied
depending on the initial pane. Code has also been moved into
pane-specific files and objects, with strict mode enabled. When calling
code from another pane (e.g., for a UI update in another pane), first
check whether the Zotero_Preferences.[Pane] object exists--if it doesn't
then nothing needs to be called.

This change breaks the word integration plugin pref overlays, which
hopefully can be rewritten to work with either overlay format.

There is a good chance that this breaks some other things in the
preferences too.

Fixes #243
Fix strict mode error downloading PDF tools
Use nicer alert for PDF tools download error