Toolbar customization

Features:

• Toggle visiblity
• Reorder
• Add custom actions in the toolbar
• Save/Load
• Add Zoom, Search and Reload buttons (depends on libxfce4ui!65 (merged))
• Update the wiki and the related Help link
• Update UCA UI to let users know where to customize visibility and that only actions related to folders are supported for now.

Must Fix:

• Remove mnemonics from the labels in the toolbar editor (see libxfce4ui!48 (04d4048d)).

#607 (closed) #621 (closed)