Add native notification box for errors on linux.

Errors will now be shown as a message box, instead of
only a console message. This fixes the user not seeing
error messages, like if the IWAD is not provided.

- Add zenity notification box call on I_Error
setup: Detect and ignore bad joystick axes.

In some cases a joystick axis can be "stuck" and have a large
uncentered value. These must be detected and ignored, otherwise the
axis can be chosen during calibration instead of the user's actual
desired axis.
Merge pull request #388 from willybarro/355_show_error_messagebox_on_linux

Add native notification box for errors on linux.
Merge branch 'master' of github.com:chocolate-doom/chocolate-doom
system: Escape special characters for Zenity.

When invoking Zenity to display an error message, some characters can
have special meanings to the shell. Escape these properly so that the
error message is always shown correctly.

This fixes #355.
strife: Fix joystick jump button.

The code to handle the joystick jump button variable was not
implemented. Check the jump button on the joystick as well as the
keyboard and mouse button equivalents when deciding whether to set
the jump bit.
joystick: Mask out bits for button axis buttons.

When a "button" is actually used as part of a button axis, don't
include presses on the button as part of the buttons field posted
in joystick events. This avoids situations where button 1 or 2
are part of a D-pad, breaking menu navigation (related to #389).