Doc: Overhauled Linux/X11, Mac OS X, and Windows platform pages.

New focus:
-Qt Installation Program
-known issues
-common content among the three platform pages
-navigation to the platform pages and their specific issues

Edited:
-removed subjective material
-combined "platform notes" and "specific issues" into one page
-moved the install, deployment, and requirements information
into one page

Individual pages still need to be edited for correctness.
Change-Id: Iaab1f8ad1aeabb772b3513f76031152bb7b81829
Reviewed-by: Martin Smith <martin.smith@digia.com>