Update vector layer properties dialog

[DelazJ]

This PR is one of the two proposals (see also #776) to update the vector properties dialog, keeping the per-tab structure in the file (though differently sequenced) and adds a new section (Note: The images below show the structure of the file):

Some information about the modifications :

• Special tools (should be renamed imho): There are transversal tools in QGIS which are hard to put under one umbrella. Among them are the color selector, the blending mode, the draw effects option, the data-defined override tool and the Save style combobox in layer properties dialog. Draw effects option and the data-defined override tool sections are not written in this PR.
• style: Following the modularity of QGIS and to avoid unneeded repetition, I remove some texts that rather belong to symbol description (a PR is coming) and pointed to such links for further information. I updated single, categorized and somehow rule-based rendering. While the other styles are hardly described, the graduated symbology needs really to be rewritten, considering all the changes occured (a bit messy).
• diagram: updated though there are some options that needs explanation
• rendering: updated (fixes Add option to force a vector layer to render as a raster #614)
• action: fixes example code formatting
• metadata, join, fields, labels, display and general chapters get few modification, mostly typo fixes or screenshots updates.
• variables: a new chapter to be written

There are various "ToDo" comments in some sections to highlight points that might be developed to improve the doc.

[rduivenvoorde]

thanks!

[rduivenvoorde]

sorry sorry :-)