issue 564: fix imports

issue 564

Empty folders are automatically skipped when browsing repository tree  (similar to github "folder jumping" feature).

GitBlit no longer uses Google charts

Formerly by default the PluginMaganer would support no proxy setting.
For servers behind firewall and HTTP proxy this would prevent installation of gitblit plugins.

When configuring a different top-left logo, we would like to allow
to have a different HxW aspect ratio image. We need then to avoid
locking down the height of the logo and just keep the existing aspect
ration as-is.

When a manager is injected through other means, (e.g. using a different
GuiceModule and injector in Gerrit) we need a way to start it without
having to necessarily passing through the injection process again.

This property allows the administrator to hide the user related part of
the top panel. This can come handy if there's no use for it (i.e. if
Gitblit runs as Gerrit plugin).

Ticket reporters should be able to specify severity whereas only those with repository push access should be able to specify priority. This also fixes a bug related to non-admin users creating tickets.

Usability bug fix

If the opacity slider was moved to the far right faster than the
animation showing the image, the user would never see the old file
because opacity got adjusted right away.

Now we first move the overlay slider to the right, so that something
is visible, and then quickly animate opacity to the current value.

Pixel difference uses CSS mix-blend-mode, which is supported currently
only on Firefox >= 32 and on Safari >= 7.1. Implementation is behind a
Javascript feature test.

For other browsers, there's a blink comparator.

Code changes:

* ImageDiffHandler now takes the page it's used on as argument. We need
  that to get labels. DOM generated is a
  little bit different (new controls).
  
* Diff pages adapted to new constructor of ImageDiffHandler.

* CSS and Javascript changes implementing the new controls, making use
  of two new static image resources. Since I felt that the new controls
  deserved tooltips, I also gave the opacity slider a tooltip: changed
  to <a>, and slider handle changed from <div> to <span>. CSS ensures
  everything still displays the same (basically display:inline-block).
  
* Supplied messages for English, French, and German for the new
  tooltips.

Tested on IE8, Safari 6.1.6 & 7.1, Chrome 38, FF 33.1 & FF 3.6.13

IE < 10 does not support gradients.

Since the new imgdiff UI heavily used gradients, this leaves IE9 and
IE8 users with a complete disfunctional UI.

Replaced all gradients by simpler means:

* Vertical handle: instead of 2px wide gradient, use a 1px background
  and a white border-right
  
* Opacity slider: No gradient but a simple border and background-color

* All radial-gradients replaced by small pre-computed PNGs

* Checkerboard background done with a small repeated PNG

With these changes, the imgdiff UI is fully functional even in IE 8.
Visual changes in other browsers are barely discernible.

Always having to drag is cumbersome. Now the slider's handle can also
be set by clicking on the slider.

Heh :-) I see the GitHub UI designers hadn't thought of that.