Make png target default for png extension.

Explanation: ext_book is a std::map so keys occur only once.
The three TARGET macros for cairo_png_trgt, png_trgt and png_trgt_spritesheet,
overwrite the ext_book value for the key "png" so the last target defined
is the one selected by default when target is deduced from extension.

With the commit we assure that for extension "png" we will use png_trgt
when the target is deduced from the png.
Translation support for Grid & Guide color warning
Keyframe Synchro : Comment about void* parameter in signal_keyframe_selected
Merge pull request #99 from d-j-a-y/master

Little fixs about Grid&Guide and Kf synchro
Add list of bones into ValueNode_BoneLink
Fix ValueNode_BoneLink
Update "Link to Bone" action
Allow to load files with  previous versions of BoneLink
Fix bug with loading "old" ValueNode_BoneLink
Remove commented code
Remove unused code
Move Panels from File to main menubar as a mainmenu item instead of submenu item.
Rename (menu) Panels to Panel
Move Properties menu item from Edit to Canvas, since it is properties of canvas.
Remove close window menuitem from File menu.
Remove unused code
Remove unused code
Remove close view action (unuse anymore)
Move (canvas) options from File to Canvas menu
Re-organize File and Edit menus

Move "Input devices..", "Setup..." and "Reset to default Setup" menu items from
File to Edit and re-arrange some items of File menu