This allows plugins to use the standard Gerrit icons.
There is now a single place were icons are stored (in
com.google.gerrit.client in gerrit-gwtui-common). Before they were
distributed over different packages. An exception are a few icons in
com.google.gerrit.client.admin and com.google.gerrit.client.diff which
cannot be moved since they are referenced from a css file in the same
package (at least I don't know how to link to the icons in
gerrit-gwtui-common from the css files).
The icon names were made consistent, duplicate icons were deleted and
the icons are also moved from src/main/java/ to src/main/resources/.
In addition the only gif icon was converted to png.
Change-Id: Ibd9475ae175de6acc7c36bf2e40f96b003f2af10
00d817a56a