Add LTR/RTL toggle in notes

Initialize the note editor with the default directionality based on the current locale, and add a single button for the opposite direction that functions as a toggle. This avoids needing two buttons.

We should also add locales to TinyMCE, but we can do that separately.

Closes #157
Remove whitespace from localized strings

Follow-up to #264
Fix Select-All on Windows (broken by d2f028d)
Remove color swatch for last-removed tag color
Fix hover effect on unedited item tags (broken in 59913cc5)
Fix Z pane brokenness with no titled items (from 53322e6e)
Fix whitespace (from 53322e6e)
Fix tag colors in the items pane in XPI builds
Zotero.File.getBinaryContentsAsync()

Currently unused, but we might need it at some point
Don't sort Year column by full date