Insert link to wiki page, web page, email

I attached the patch containing the implementation for insert link feature.
Anca will add the new implementation for the unlink command and enabling/disabling the buttons from the toolbar.

A few general and short comments about the patch (the submitted one and also the intermediary versions of it that I've seen and commented):

• a few notes on the architecture approach:

• weak usage of a controller to transform view calls (UI calls) into server aync calls - which led to specific, non-reusable serverside code written to serve the particular purpose of a specific ui action
• too much duplicate code (the initial service implementation (not present in this patch), the links tabs)
• useless storage of data in member variables, especially as this happened in the context of the client code
• strange approach for a couple of problems (e.g. filling the dropdown boxes on selection change), messed up responsibilities (e.g. LinkDialog responsible for initialization and update of its child tabs)
• as a general impression, not enough overall view, local analysis of situations instead of a contextual approach and integration with the rest.
• otherwise good, clean, non-hacky approach. Easy to understand the general idea and relatively easy to work on top of it. Also, working, tested code.

• codestyle, procedures:

• good, strict follow of formatting rules
• awareness and follow of checkstyle rules, but missing or useless comments someplaces.
• no comments whatsoever in code, which should be there to help understand the logic or necessity of some parts

It would be good to see more agile-style approach of problems: incremental development, starting with minimal functionalities / code, then iterating and adding more and more, with review of the whole landscape and potential refactor at each iteration.