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).
Build fix with gtk3
Workaround for a crash in gtk3 version of xfce4-indicator-plugin
Fix to the previous commit
Setting gtk3 StyleContext class in wrapper.

(reworked:
2820b5bc195adfc49490d9001f8f4e8677140203
    Added panel class for theming.
3557ce18a3f0dfa88ebaa8c2f69f80e12fa69c5f
    Use both .panel and .xfce4-panel style context classes
from nick/gtk3 branch)