Fix problem with items list scrollbar size between views

Caused by 2cd45be191, though not sure why it's happening.
Download locally existing remotely modified files in on-demand mode

If a file doesn't exist locally it won't be downloaded until opened, but
if a file does exist locally and has been modified remotely the latest
version will be downloaded at sync time regardless of the file sync
mode. (If a locally existing file has been updated remotely, just
opening it won't currently download the latest version. You have to
sync. This may change in the future.)
Remove debug line and update error message
Fix update marking for uploaded file versions that exist remotely

If a file version existed on the server, the uploaded attachment wasn't
marked as updated until the next sync.
Remove redundant line
Only show annotation toolbar on pages that already have annotations
Unbreak adding new items. Broken by dbf6bc0ee1fb216953be43176011f93627ca2208.
Merge pull request #274 from aurimasv/newItem-fix

Unbreak adding new items
Fix code style

Refs #274
Fix missing "Deleted" in CR window

Addresses #273
Remove reference to missing .dtd file
Sort by title after creator and date, if not primary sort

Addresses #275
Use dateAdded rather than dateModified for final fallback sort

Addresses #275
Fix collections-containing-an-item highlighting in Fx22+
Fix error selecting separator above Group Libraries

This is super annoying, but I can't seem to stop the separator row from
being selected in seltype="single" mode (where isSelectable() isn't
called). Until we have a better solution, at least clear the items list
and avoid an error.
If Year column is visible and not Date, don't sort by full date

Otherwise use full date, even if Date column is hidden

Addresses #275
Sort empty titles last when title isn't primary sort

When sorting by Title, empty titles get sorted to the top of the items
list for visibility, but when sorting by another column and using the
title as a secondary/tertiary sort, empty titles should get sorted last
so that new empty items go to the end of the list rather than the
middle.

This is a little weird, and the alternative would be to just always sort
empty titles last even when sorting by Title, but this preserves the
current behavior for Title sorting. (Before f0f6772b01a titles weren't
used for secondary sorting at all, so there's no precedent for
title-sorting behavior when sorting by another column.)

Addresses #275
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