SciDAVis - Improvements SVN-temp

- Added some information about LabPlot collaboration and related resources to
Contributing/Coding.

- Changed wording of Windows download instructions which looked a bit strange,
considering the age of the released manual.

- Improved layout of Linux section on the download page and added Zenwalk to the
list of distributions known to contain SciDAVis.
- Removed reference to separate translation releases. We're currently not doing
these.

- Documented classes Fit and Folder.
- Documented some recently-added methods.
- Improved and clarified some existing documentation; in particular, emphasized
the move from 1-based to 0-based indices in new APIs.

- Fixed building with searching for updates enabled. Thanks to Miquel Garriga for spotting this.

- Added changing of the active folder to Python API, allowing scripts to add MDIWindows to arbitrary folders.

- Added command-line option for debug/release build selection.

- Made creation of subfolders available to Python scripts. The method is
meant to be forward-compatible to future SciDAVis versions built entirely
on top of AbstractAspect: folder.addChild(Folder("subfolder")).

- Fixed bug #2894751 (Column mix up in formula when underscore in name).

- Fixed bug #2979848 (Converting a curve between lines/points and bars broken).