config: SOURCE_BROWSER_ONLY

When set to YES, the new config option,
will only generate the highlighted sources.
Information is provided through SOURCE_TOOLTIPS.

It helps if you only use doxygen as a source browser and want
a to have a quicker html output.
Also, so far, doxygen can either
a) generate the docs
b) a) + source browser
but no way of having only the source browser.

Tested on libdrm:
SOURCE_BROWSER_ONLY = NO
  Elapsed (wall clock) time (h:mm:ss or m:ss): 0:14.27
  9285 files in html dir

SOURCE_BROWSER_ONLY = YES
  Elapsed (wall clock) time (h:mm:ss or m:ss): 0:02.65
  717 files in html dir

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>