Separate UNCHECKEDBOX from SQUARE. Move more images to SVG

UI/Theme.cpp

@@ -205,7 +205,7 @@ void UpdateTheme() {
 #endif
 
 	ui_theme.checkOn = ImageID("I_CHECKEDBOX");
-	ui_theme.checkOff = ImageID("I_SQUARE");
+	ui_theme.checkOff = ImageID("I_UNCHECKEDBOX");
 	ui_theme.whiteImage = ImageID("I_SOLIDWHITE");
 	ui_theme.sliderKnob = ImageID("I_CIRCLE");
 	ui_theme.dropShadow4Grid = ImageID("I_DROP_SHADOW");

UI/UIAtlas.cpp

@@ -51,6 +51,7 @@ static const std::string imageIDs[] = {
 	"I_STICK_LINE",
 	"I_STICK_BG_LINE",
 	"I_CHECKEDBOX",
+	"I_UNCHECKEDBOX",
 	"I_BG",
 	"I_L",
 	"I_R",