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

chrome.manifest

@@ -49,8 +49,13 @@ skin		zotero	default		chrome/skin/default/zotero/
 
 overlay	chrome://browser/content/browser.xul	chrome://zotero/content/statusBarOverlay.xul appversion<4.0
 overlay	chrome://browser/content/browser.xul	chrome://zotero/content/overlay.xul
-overlay	chrome://zotero/content/preferences/preferences.xul			chrome://zotero/content/preferences/preferences_firefox.xul application={ec8030f7-c20a-464f-9b0e-13a3a9e97384}
+
+overlay chrome://zotero/content/preferences/preferences.xul chrome://zotero/content/preferences/preferences_firefox.xul application={ec8030f7-c20a-464f-9b0e-13a3a9e97384}
 overlay	chrome://zotero/content/preferences/preferences.xul#cite	chrome://zotero/content/preferences/preferences_firefox.xul application={ec8030f7-c20a-464f-9b0e-13a3a9e97384}
+overlay chrome://zotero/content/preferences/preferences_general.xul chrome://zotero/content/preferences/preferences_general_firefox.xul application={ec8030f7-c20a-464f-9b0e-13a3a9e97384}
+overlay chrome://zotero/content/preferences/preferences_export.xul chrome://zotero/content/preferences/preferences_export_firefox.xul application={ec8030f7-c20a-464f-9b0e-13a3a9e97384}
+overlay chrome://zotero/content/preferences/preferences_advanced.xul chrome://zotero/content/preferences/preferences_advanced_firefox.xul application={ec8030f7-c20a-464f-9b0e-13a3a9e97384}
+
 overlay chrome://mozapps/content/downloads/unknownContentType.xul chrome://zotero/content/downloadOverlay.xul
 
 style	chrome://browser/content/browser.xul	chrome://zotero/skin/zotero.css