replace fight_on_without_leader with defeat_condition

[side] now has an attribute defeat_condition which can be "no_leader"(default), "no_units" or "never"
"no_leader" behaves liek it behaved before.
"no_units" behaves like fight_on_without_leader=yes behaved before
"never" is new and causes the side to never get defeated, this usually means the scenariodesigner has to end the scenario with [endlevel]
Use xvfb on travis

As instructed on http://docs.travis-ci.com/user/gui-and-headless-browsers/
Disable some failing tests

These tests may no longer be correct. Their author is looking into them.
add "always" defeat_condition
rename remove_from_carryover_on_leaders_loss_

to remove_from_carryover_on_defeat_
add map_location rotate functions, and tests
in debug mode, add team strings as well as names for hidden sides.

This brings the team names in line with the player names and other
behavior in mp_connect in debug mode. For instance there are other
lines like this:
https://github.com/wesnoth/wesnoth/blob/dbf3331e9a99e75ca0c16009169eebeddb9c1db1/src/multiplayer_connect_engine.cpp#L155

From inspecting the save files generated while testing this issue,
it's not clear if it could have actually caused a problem in game,
but it addresses an issue raised by SlowThinker on the bug tracker:

https://gna.org/bugs/index.php?21800

and does indeed fix a minor shortcoming of debug mode in mp connect
engine.
get_relative_dir may take a const & argument
fixup header to restore gcc builds
Always indicate that faction was chosen when placing player.

This fixes bug #21977.
Minor style improvements.
add a "radial mode" option to get relative dir. (just for kicks)

Adds a mode for get relative dir which should give radial symmetry,
using vector ops. This might be useful in some application, or to
help debug.
move default relative dir mode to .cpp file, for ease of testing
Update changelog add 'defeat_condition'
don't load null hotkeys

this prevents a possible segfault because otherwise add_hotkey calls hotkey::set_scope_active(new_scope) where new_scope is SCOPE_COUNT.
add wml relative dir query tag, and interactive test scenario
add map location rotate wml interface
fixup radial mode get_relative_dir
get_relative_dir to use radial_symmetry mode, until we fix default

Revert this when we have fixed up the default mode.
Update VC project for b4ea561e37