QDoc: Introduce a variable to set table of contents depth.

-"HTML.tocdepth" variable controls depth value.
-setting to "0" disables table of contents.
-sections 3 and 4 usually don't have descriptive titles
 to warrant their listing in the table of contents.
-table width and CSS (online and offline) don't support wide entries.
-Config class' getInt() function now returns -1 if a variable is
 not set.
-for Qt 5 and projects which use html-config.qdocconf, tocdepth is
 set to "2".
-added variable documentation.

Task-number: QTBUG-38967
Change-Id: Ibd612f5b846ecb9c4b575e7ac11605c6efd2b77c
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>