flac: STREAMINFO is not necessarily the first METADATA_BLOCK

Close: #8830
flac demux: fix picture leak
flac demux: fix seekpoint leak
skins2: fix nested panels wrongly positioned if not the first child

plus factorise some code.

This fixes a bug reported on the forum at
http://forum.videolan.org/viewtopic.php?f=26&t=111572
skins2: fix documentation error

as reported on the forum at http://forum.videolan.org/viewtopic.php?f=26&t=111572
skins2: fix stray line in Qt menu used by skins2
macosx: dialogs provider: fix crash for question dialog and also improve robustness of other dialogs
contrib: gettext: fix osx patch
skins2: improve fullscreen behaviour

At the core level, the 'f' hotkey and the double click are managed in a
slightly different way. The former sets up both the playlist and the vout
"fullscreen" variables whereas the latter only sets up the vout variable.

This patch first relies on the vout state if any, and falls back to the
playlist state. This ensures that the fullscreen button always works
consistently for the user. Otherwise, user needs to press twice instead
of once when the playlist and the vout are no longer in sync (after a double
click)
Revert "contrib: fix bghudappkit for xcodebuild of vlc project"

This reverts commit acf5397b37d648b8df13eb3a0d1bee14d8e3ae2c.

Conflicts:
	contrib/src/bghudappkit/rules.mak
osx: fix crash with --no-media-library
vlc.desktop: add missing --started-from-file option (fixes #8839)
OSX: fix crash when no SD are found
skins2: extend xoffset and yoffset parameters

Till now, these two parameters only had a meaning for geographical positioning.
As explained at http://forum.videolan.org/viewtopic.php?f=26&t=111572,
extending xoffset and yoffset for the general case would help a lot skin
developpers easily position an object of unknown size around a given point :
for instance,
    x="50%" xoffset="-50%" would center an object horizontally.
    x="100%" xoffset="-100%" would set an object to the East

This patch doesn't modify the existing behaviour. It just implements a new
feature for the case where it was a silent no op.

x,y, width, height percentages are computed against the parent container
xoffset and yoffset percentages are computed against the current object

TODO: this approach could be generalized to all objects (need to extend the dtd)
skins2: fix documentation wrt to xoffset, yoffset, xmargin, ymargin
skins2: update documentation for xoffset/yoffset
skins2: add width/height parameters to slider

These parameters will allow to create a slider which adapts to the display.
width/height parameters will be optional. If set, the initial slider will
be resized to meet these requirements.

A main use case will be the possibility to create a fullscreen timeslider
that automatically spans the whole screen whatever the screen resolution.
e.g syntax would be <Slider width="100%" ....>
skins2: update documentation (width/height for image and slider)
invert: check supported chroma (fixes #8840, fixes #8841)
Qt: ExtensionCopy: fix memleak