retina support: convert logical to physical pixels for default_border_width
Align lower line of bar decoration to border width

In normal border style, two horizontal lines are drawn to outline the
bar decoration. The lower line leaves a gap to the left and right to
align with the border. This gap was hardcoded to 2 pixels. Now it
matches the current border width, if any.

In stacked layout, only the bottommost bar is aligned. In tabbed layout,
no gaps are left in order to close the outline and make the notion of a
tab more clear.
Treat everything up to 120 dpi as 96 dpi (Thanks jefvel)

See the comment in the source for rationale.
Parse tray_output as a word, not string (Thanks Peter)

This makes it robust against trailing whitespace.

fixes #1352
Allow to validate the config file without X.

We're going to call parse_configuration() very early if -C is given on
the command line. Instead of the previous "only_check_config", which has
been a global variable, we now simply pass use_nagbar as false if we're
just validating.

This causes the whole parsing to run without X and of course without
starting nagbar and displaying the errors to standard out/error instead.

The return code of parse_configuration() is now a boolean which
represents whether an error occured during parsing and the programs exit
code is returned accordingly.

Although the config parser still has a lot of side-effects, we now can
parse without the need to have an XCB connection. A nicer implementation
would be to just set the new font and load it just after we're done
parsing, but to ensure we don't break functionality we just load a dummy
FONT_TYPE_NONE if XCB isn't available. The main reason for going this
route is that it's a bit difficult to test fonts in a distribution
agnostic way without bundling fonts with i3 (or Xdummy to be more
exact).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Properly handle windows unsetting WM_TRANSIENT_FOR (Thanks Janus)

fixes #1351