Feature: Group liveries, and livery window usability enhancements. (#7108)

* Change: Replace checkbox in livery selection window with Default option in drop down selection.

This reduces clutter in the UI and allows for primary/secondary colours to independently follow the default scheme if desired.

* Feature: Add vehicle group liveries.

media/extra_grf/openttd.nfo

@@ -14,6 +14,8 @@
 // allowing it to be used.
 //
 
+//@@WARNING DISABLE 60
+
 //
 // Number of sprites, it is wrong, but GRFcodec automagically gets it right.
 //

media/extra_grf/openttdgui.nfo

@@ -7,7 +7,7 @@
 // See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
 //
    -1 * 0	 0C "OpenTTD GUI graphics"
-   -1 * 3	 05 15 \b 175 // OPENTTD_SPRITE_COUNT
+   -1 * 3	 05 15 \b 179 // OPENTTD_SPRITE_COUNT
    -1 sprites/openttdgui.png 8bpp  66    8  64  31 -31   7 normal
    -1 sprites/openttdgui.png 8bpp 146    8  64  31 -31   7 normal
    -1 sprites/openttdgui.png 8bpp 226    8  64  31 -31   7 normal
@@ -183,3 +183,7 @@
    -1 sprites/openttdgui.png 8bpp 440  440  20  20   0   0 normal
    -1 sprites/openttdgui.png 8bpp 466  440  20  20   0   0 normal
    -1 sprites/openttdgui.png 8bpp 490  440  20  20   0   0 normal
+   -1 sprites/openttdgui_group_livery.png 8bpp   0    0  20  20   0   0 normal
+   -1 sprites/openttdgui_group_livery.png 8bpp  21    0  20  20   0   0 normal
+   -1 sprites/openttdgui_group_livery.png 8bpp  42    0  20  20   0   0 normal
+   -1 sprites/openttdgui_group_livery.png 8bpp  63    0  20  20   0   0 normal