Set color of scrollbar/dropdown button symbols and checkmarks to white

Before this patch, the symbols were rendered black on dark background.

Previous images were edited like this:

1. The colors were inverted with GIMP's “linear inversion” method.
2. Image files were optimized using “optipng -o7 -zm1-9 -strip all”.

Co-authored-by: ROllerozxa <rollerozxa@voxelmanip.se>
Co-authored-by: rubenwardy <rw@rubenwardy.com>

builtin/mainmenu/dlg_config_world.lua

@@ -245,7 +245,7 @@ local function get_formspec(data)
 
 	return retval ..
 		"tablecolumns[color;tree;image,align=inline,width=1.5,0=" .. core.formspec_escape(defaulttexturedir .. "blank.png") ..
-			",1=" .. core.formspec_escape(defaulttexturedir .. "checkbox_16_white.png") ..
+			",1=" .. core.formspec_escape(defaulttexturedir .. "checkbox_16.png") ..
 			",2=" .. core.formspec_escape(defaulttexturedir .. "error_icon_orange.png") ..
 			",3=" .. core.formspec_escape(defaulttexturedir .. "error_icon_red.png") .. ";text]" ..
 		"table[5.5,0.75;5.75,6;world_config_modlist;" ..

src/client/clientlauncher.cpp

@@ -138,6 +138,7 @@ bool ClientLauncher::run(GameStartData &start_data, const Settings &cmd_args)
 
 	guienv = m_rendering_engine->get_gui_env();
 	skin = guienv->getSkin();
+	skin->setColor(gui::EGDC_WINDOW_SYMBOL, video::SColor(255, 255, 255, 255));
 	skin->setColor(gui::EGDC_BUTTON_TEXT, video::SColor(255, 255, 255, 255));
 	skin->setColor(gui::EGDC_3D_LIGHT, video::SColor(0, 0, 0, 0));
 	skin->setColor(gui::EGDC_3D_HIGH_LIGHT, video::SColor(255, 30, 30, 30));