Remove unused icons and ensure template icons
Use NSImageCell for rendering sidebar effect

This removes need for own rendered highlighted sidebar icon.
Resolution independent PDF icons (with SVG source)

This is replacement for icons by Alexis Hildebrandt. I tried to be close as
possible to Alexis design, however there are following changes:

(1) Icons are now PDF, no need for @2x version, less files
(2) SVG source, instead Pixelmator which is proprietary paid software
(3) Icons are pixel aligned to non-Retina pixel boundaries (look sharp)
(4) Folder & remote (cloud) look are more inline with OSX default icons
SVG scale independent log (WebKit based) icons

Also removed old Acorn sources and Pixelmator sources. Icons were made to
preserve style of previous ones. No need for source as icons are SVG.
Pixel aligned PDF file icons (13 px height)
Merge branch 'patch/fix-sidebar-template-icons' of github.com:nanoant/gitx into nanoant-patch/fix-sidebar-template-icons
Add pull request #280 to the change log