[GUI] Improve online help

[tangobravo62]

• Add help/info texts for each page (e. g. mouse-overs, question mark button).
• Link/integrate the prosEO Wiki.
• Explain user roles (mouse-over hints for roles)

[emelchinger]

Implemented, explanation like description in Wiki.

[emelchinger]

Implemented.
A question mark in the header of each page links to the corresponding wiki article.
Mouse over help for menu entries, roles and action signs (like +, -, x).