gui2/tmessage: Add missing standard padding around image cell
gui2/ttransient_message: Add missing standard padding around image cell
gui2/ttransient_message: Hide title and image widgets when unused

When the title or image widget labels are left empty, the widgets
continue to take up space in the grid, which is particularly conspicuous
with our current border,border_size=all,5 convention.

Commit 037ec6af8b307c20de860d670744deab309f7b23 exposes the underlying
problem quite clearly in most dialogs because the image widget's cell
takes up extra space to the left of the dialog while empty -- see the
Objectives popup for an example.

Hiding the relevant widgets when missing label values is trivial to do,
even if it requires additional boilerplate in the
gui2::ttransient_message case (no existing pre_show() method override),
so it should be a good solution as any until cell borders gain the
ability to become null when the contained widget is otherwise unused.

NOTE: I had to move the inclusion of gettext.hpp around a bit in order
to avoid name collisions with Boost headers introduced along with the
two new GUI2 inclusions in transient_message.cpp. The _ gettext
shorthand macro is truly a PITA to have near some of the most
complicated Boost libraries.
gui2/tmessage: Hide title and image widgets when unused

When the title or image widget labels are left empty, the widgets
continue to take up space in the grid, which is particularly conspicuous
with our current border,border_size=all,5 convention.

Commit b9b199f4f19a414813c28db68e903b9bb5306101 exposes the underlying
problem quite clearly in most dialogs because the image widget's cell
takes up extra space to the left of the dialog while empty -- see the
"you have not started your turn yet, do you really want to end your
turn" prompt for an example.

Hiding the relevant widgets when missing label values is trivial to do.
In the gui2::tmessage case it required a minor internal refactoring to
keep a find_widget<>() reference around for a second method call for
each widget. This should be a good solution as any until cell borders
gain the ability to become null when the contained widget is otherwise
unused.
updated Slovak translation
updated German translation
Support SDL2's mouse wheel event in mouse_handler_base.

Allows to scroll the map with the wheel.
Wrap old SDL code in a preproc directive.
Support for SDL2's mouse wheel event in the GUI1 slider.
Wrap old SDL code in a preproc directive.
Support for SDL2's mouse wheel event in the GUI1 scrollarea.
Support for SDL2's mouse wheel event in the GUI1 scrollbar.
Wrap old SDL code in a preproc directive.
Switch vertical scrolling directions.
Support for SDL2's mousw wheel event in the palette manager.
Wrap old SDL code in a preproc directive.
Make a constant variable a reference.
Support for SDL2's mouse wheel event in GUI2.
Wrap old SDL code in a preproc directive.
sound: Include SDL_Mixer channel layout in debug output