travis: skip apt-get upgrade on the CXX11 build

it doesn't run the test executable anyways
fix copy assignment operator for unit

The new version is exception safe, using copy and swap, and should
be correct with reference counting.
travis: only run apt-get upgrade when the cpp unit tests run
Fixed bug #23048

bug #23048: EI: Evacuation - ill-formed terrain modification causes
horizontal line
Move the blend_rgb() method to sdl/utils and rename it to blend_rgba.

Added support for an alpha value.
Add support for setting the drawing content of a floating label.
Add support for setting the drawing content of tooltips.
clean up game_display (prefer local variables over global variables)
rename events::handler to events::sdl_handler

This is much less confusing, as we have many different types of
event handlers, and will probably have more soon...
remove some dead code

This code isn't needed after referencing counting was implemented
for units.
Fix https://gna.org/bugs/index.php?23065
play_controller: favor local variables over global
move some game-save code from event manager to play_controller

This code doesn't need to be here, it is confusing that it is as it
violates separation of concerns.
make "reports" not a singleton object

This is necessary to make game_events not a singleton.
Allow removing labels by handle.
Support for tooltips bound to a map label.
Enable label tooltip support from action wml.
Call recalculate_shroud for the labels when scrolling.

This is needed to make sure the tooltip location is updated.
Add some tooltips to the test scenario's labels.
fixup 0459f8a57411b35b61d92fead7214ff3514c957c

Some compilers on travis wanted the type to be more explicit.