style: Avoid conflict with GTK widget names

GTK style provider loads the theme on top of the selected CSS file. To ignore the theme, element names must be different than the predefined ones.

data/style.css

@@ -1,10 +1,10 @@
-layout {
+sq_view {
     background-color: rgba(0, 0, 0, 255);
     color: #ffffff;
     font-family: cantarell, sans-serif;
 }
 
-button {
+sq_button {
     color: #deddda;
     background: #464448;
     border-style: solid;
@@ -13,7 +13,7 @@ button {
     border-radius: 3px;
 }
 
-button:active {
+sq_button:active {
     background: #545256;
     border-color: #716e78;
 }