Better icons for preferences window

[dstillman]

The current icons are bundled with Firefox and may not all even appear on all platforms.

The Export pane icon isn't even the right size (24px vs. 32px for the others).

In general, we need a source for larger high-quality icons, since famfamfam is limited to 16x16px.

[dstillman]

(In [1203]) Closes #359, Ability to store Zotero directory outside of Firefox profile directory

Customizable in the preferences

Also improves handling of Zotero startup errors, adding the ability to customize the tooltiptext of the status bar error icon and to have ZoteroPane.toggleDisplay() run a customizable error function (it no longer opens an empty and broken Zotero pane). For example, a missing Z directory now prompts the user to locate the directory.

Various code parts now check for the Zotero object and Zotero.initialized before trying to do stuff, which should cut down on redundant error lines in the console and generally make things cleaner.

Refs #542, Better icons for preferences window -- need icon for Advanced prefpane

[dstillman]

(In [1292]) Fixes #542, Better icons for preferences window

Also better spacing of prefpane buttons