Announce current chooser item after edits in search field

This puts the VoiceOver user experience in OakChooser on par with
Spotlight in Yosemite which also announces currently selected item
after every edit in the search field. This provides the user with
feedback so they know whether the search result changed or did not
change after typing/deleting in the search field.

A manual AXSelectedRowsChanged notification is used - see
http://lists.apple.com/archives/accessibility-dev/2014/Aug/msg00024.html
for discussion about this.

This change is Yosemite-only. While pre-Yosemite OS could probably be
supported in the way it is now for the case when the user moves the
table results with arrow up/down, I did not have time to fiddle with it
and given that Yosemite will be released soon, I am not sure it will be
worth the effort.
Ensure commit dialog attaches to the correct window
Improve mate usage information
Rename (template) images to have a ‘Template’ suffix
Don’t specify bezel style for borderless image buttons

Also skip setting the template image property, since all the images have the ‘Template’ extension.
Change gutter view data source API

This changes behavior slightly as the column width is now decided by the data source rather than based on the widest image.

The gutter view expects the data source to scale images to fit in the available space, since the data source knows about (ideal) line height in the text view and also because scaling the images may lose precision in the image ratio, so scaling it later (if line number font is increased) would give a suboptimal result.
Rename search gutter view image to match name used for the mark
Use mark’s name as image name

This way new marks do not require code changes.
Add padding to the search gutter view image

Since we now scale images to full width and/or height of available space, padding is required for some of them.
Update bookmark gutter view images

These are now optimized for a width of 13 points, which is the width when using the default 12 point Menlo font.
Ignore unknown file types when finding document’s file type

The problem is if the user sets a certain file type to open with grammar X and then uninstalls X, there will still be a setting binding the file type to X which means we skip trying to find another suitable grammar.
Cleanup code dealing with marks

Previously marks were lost when disposing a document.
Fix display path when searching open files