Loading external gtk3 panel plugins in gtk2 panel.

Requires a gtk3 version of libxfce4-panel and wrapper to be compiled
from another branch.

This commit is only a quick hack to make the plugins to load.
It should be reworked (starting from better naming) for a release
version.
Added GTK+-3 version of libxfce4panel.
Made libxfce4panel more in sync with gtk3 branch.
Added missing .pc file.
Wrapper ported for both Gtk+-2 and Gtk+-3
Change WRAPPER_BIN name to match the wrapper.
Loading external gtk2 panel plugins in gtk3 panel.

Addeding X-XFCE-API to the plugin .desktop files to determine which
wrapper to use. This falls back to 1.0 wrapper for compatibility with
the current plugins.

Conflicts:
	panel/panel-module.c
	panel/panel-plugin-external-wrapper.c
Fixed drawing code in panel-image and arrow-button

- top-left coordinate at 0,0, not alloc->x,alloc->y
- use gdouble instead of gint in _draw
- use GtkStyleContext
- fix uninitialized alloc variable
panel-image: use style-update signal rather than style-set.

Doesn't seem to make any difference here but style-set is deprecated
since Gtk+ 3.0.
PanelImage: better support for non-square icons.

I thought this might be the reason for panel buttons growing infinitely.
It turned out to be something different but this change still improves
the sizing and makes it consistent with xfce_panel_pixbuf_from_source_at_size.
PanelImage: reworked workaround for GtkButton sizing issue.

It should now work in all scenarios (custom buttons, can_focus=FALSE etc.)
PanelImage: fixes to previous commit and cleanup.

Correction has to be done in all cases, not only when the requested
size is derived from allocation.
ArrowButton: sizing fixes.
PanelImage: allow scaling the image down (below allocation)

Some containers (GtkBox(?)) refuse to allocate their child items below
minimum requested size, even if that results in violating their own
allocation.

Reducing the minimum size allows these containers to iteratively
reduce the size of the embedded PanelImage.

When priv->size > 0 minimum size must be equal to the natural size.
Otherwise icons displayed in menus will be too small (menus use
a minimum size).
PanelImage: better handling of minimum sizes.
ArrowButton: better handling of minimum sizes.
PanelPlugin: workaround for transparency issues.

It disables plugin transparency completely (breaking custom background
colors, bitmaps and alpha settings). But at least it makes the panel
usable with "system style" background settings.

How to fix it properly?
External plugins work fine but they have their own API for setting
the background style.
xfce4-panel lib: workaround for ctx menus with scroll buttons.
PanelImage: do not reload fixed-size icons on style-updated.

Style-updated triggered a lot of flickering in the applications menu
on mouse hover events.

Are there any use cases where reloading the icons is needed?
Quick fix of compilation errors after merge of nick/gtk3 branch.

Gtk3 plugins (e.g. xfce4-indicator-plugin/master) are not recognized
(treated as gtk2 ones).