Some tweaks to coverage support

Rename the configure option to --enable-coverage, and make
it quiet by default.
Test some gsettings corner case

Setting a strv to NULL was not tested before.
Improve test coverage for GPropertyAction
Slightly improve coverate for notification tests
contenttype tests: better assertions

I recently had to track down why these tests failed. Turned
out that some rogue package on my system had installed mime
types that declared all files with 3 letter names to be
'chemical/x-turbomole-vibrational'.
This change will make it more obvious what is going on by
mentioning the mime types in the assertion message.
Improve gsettings test coverage

Deal with corner cases around delay.
Fix a !srcdir problem with installed tests

When installing scripts which are not generated in a
srcdir != builddir build, $< will have ../.. goo in it.
So we need to strip the directory parts before using
the filename to construct a new path.
Fix a typo
Simplify coverage rules

This works just fine, no need for the manual removal of the
info file.
More test coverage for settings backends
GKeyfileSettingsBackend: Use a directory monitor

GKeyfileSettingsBackend was using a file monitor to monitor
the parent directory of its keyfile.
More settings backend tests

This adds some more test coverage around writability changes
in the keyfile and delayed settings backends.
GVariant: Add g_variant_parse_error_quark()

Most GErrors, such as GSomethingError, have a function to get
their quark that looks like g_something_error_quark(),
so bindings (such as gtkmm) would expect GVariantParseError
to have g_variant_parse_error_quark(). Instead this had
g_variant_parser_get_error_quark().
This deprecates the old function and adds the correct one,
making life easier for gtkmm (and maybe others).

https://bugzilla.gnome.org/show_bug.cgi?id=708212
Use POSIX-specified <poll.h> over <sys/poll.h>

POSIX specifies that <poll.h> is the correct header to include for
poll(), so let's do that instead.

https://bugzilla.gnome.org/show_bug.cgi?id=141251
GVariant: add way to print context of parse errors

This was a feature intended from the very beginning that somehow never
got written.  It's a way to replace these sort of error messages out of
the GVariant parser:

  1-2,10-15:unable to find a common type

with something in the style of the Vala compiler:

  unable to find a common type:
    [1, 2, 3, 'str']
     ^        ^^^^^

https://bugzilla.gnome.org/show_bug.cgi?id=715028
gsettings tool: print GVariant errors in context

Use g_variant_parse_error_print_context() to format the error message
from the GVariant parser.

https://bugzilla.gnome.org/show_bug.cgi?id=715028
gapplication(1): print GVariant errors in context

Use g_variant_parse_error_print_context() to format the error message
from the GVariant parser.

https://bugzilla.gnome.org/show_bug.cgi?id=715028
gdbus tool: print GVariant errors in context

Use g_variant_parse_error_print_context() to format the error message
from the GVariant parser.

There is a slightly dubious interaction with the "parse me harder"
functionality here.  We're probably going to have to deal with that
separately.

https://bugzilla.gnome.org/show_bug.cgi?id=715028