Workaround for accessibility of OakChooser's search field

Currently for VoiceOver user when the text cursor is at the end of the search
field (which is 99% of the time) and the user wants to navigate the results
in the table view using arrow down, then after each arrow down, VoiceOver first
plays a "end-of-text" sound, then announces whole contents of the search field,
and only then announces the newly selected search result. (The same happens
when text cursor is at the beginning of the search field and user presses
arrow up.)

This is probably a bug in the way AppKit handles text field accessibility - see
http://lists.apple.com/archives/accessibility-dev/2014/Feb/msg00019.html

I also reported this as <rdar://problem/16271507>

I hereby present a hacky workaround for this - a subclass of NSSearchField
that tricks accessibility into thinking there is one extra space before and
after the text in the search field. Therefore VoiceOver will not think the user
is at the end of text when they actually are, and therefore will not play the
"end-of-text" sound and announce whole contents of the search field before
getting to the information user wants (the newly selected search result).

While this is a bit hacky, along with the previous commit it allows VoiceOver
to have the same instant great experience as a sighted user when filtering
and browsing results in OakChooser. So I think it's worth the hackiness until
there is a better alternative.

There is only last issue, as currently VoiceOver is very chatty when the
OakChooser window appears: first says "Go to file", if user interrupts it with
an action (i.e. moving VO cursor or typing a character), then VoiceOver starts
telling the whole title of the window, and only after user interrupts it again
does VoiceOver say or announce only the user actions (including search results
table selection change).

So the simpliest if the user wants to start navigating the table items
immediately after showing the OakChooser's window (i.e. without entering
a search term) is to, after showing the OakChooser (e.g. with ⌘T or ⇧⌘T),
quickly type any letter and immediately delete it with backspace and then
use arrow down/up.