git-gui: Use more modern looking icons in the tree browser

git-gui: Use more modern looking icons in the tree browser

This is a replacement of all of the icons in our tree browser
window, as the prior icons just looked too 1980s Tk-ish.  The
icons used here are actually from a KDE themed look, so they
might actually be familiar to some users of git-gui.

Aside from using more modern looking icons we now have a special
icon for executable blobs, to make them stand out from the normal
non-executable blobs.  We also denote symlinks now with a different
icon, so they stand out from the other types of objects in the tree.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>