Backtrace via call detail

Hello,

Changes from v3: Instead of writing the backtrace as Array, the backtrace is
now recorded as a list of stack frame nodes with optional stack frame details
(the scheme is below).

This patch implements backtrace recording during tracing, and adds support in
'apitrace dump' and QApitrace. Backtrace is obtained via platform-specific
functions (and, internally, in platform-specific format). Then it is parsed to
produce an std::vector of stack frame structs: { char *module, *function,
*filename, *linenumber, *offset } (some fields may be NULL) and is written
into the trace file in the Enter call section as a call detail:

BACKTRACE
    FRAME
        MODULE "module"
        FUNCTION "Foo"
        FILENAME "foo.cpp"
        LINENUMBER "1234"
        OFFSET "0xSDF"
    FRAME
        FUNCTION "Boo"
        // no filename line info available for this frame
END_BACKTRACE

A platform-dependent mechanism is provided to specify a set of traced
calls for which backtraces will be recorded. It is possible to specify
either function names, or prefixes of names by appending a '*' (e.g.
"glUniform*").

On Android the backtrace is retrieved from Dalvik via libdvm functions
imported at runtime.
Function set is specified in /data/apitrace.fnames, one per line.

On Linux the backtrace is retrieved via glibc backtrace(), and will not always
yield filename:linenumber information.
Function set is specified via APITRACE_BT_FUNCTIONS environment variable.

On other platforms, obtaining a backtrace is not implemented by this patch.
Remove FunctionSig::backtrace member.

Backtraces are really disjoint from signature.

This puts back const keyword in most FunctionSig usages.
Cleanup the backtrace representation.

Use strings and unsigned, instead of string values and unsigned values.
Fix windows build.
Use ids for frames.

So that we don't need to send the same frames over and over again.
Fix Android build.
Re-add backtrace_is_needed() call.

Removed by mistake during refactoring of commit
69909e3853c23d0ce062b6c9f232191970da9aee
gui: Make "Details View" the default pane.

Instead of the "Backtrace".
common: Cleanup backtrace.

There was a bool field that was caching results of 'is_backtrace_needed'
lookups.   Now that field is gone, and 'is_backtrace_needed' is called
repeatedly.  I guess it's not that bad (it boils down to a lookup in an
std::set), but that function was also printing notices for functions
with enabled stack trace recording, and now those messages are printed
repeatedly as well.  The following patch removes the notices completely
and slightly cleans up the surrounding code.
Merge branch 'backtrace'

Conflicts:
	common/trace_writer_local.cpp