UI: Fix scrollbar handle alignment for Dark theme

Fixes an issue when it was hard to drag by mouse the scroll handle in
Dark theme if scroll handle size is close to full size
(https://obsproject.com/mantis/view.php?id=466) or close to minimal
size.

Also, the mouse pointer was misaligned to the scroll handle when
moving: the scroll handle always moves faster than mouse pointer itself
and finishes first.

UI/data/themes/Dark.qss

@@ -150,6 +150,7 @@ QGroupBox {
 QScrollBar:vertical {
     background-color: rgb(58,57,58); /* dark */
     width: 14px;
+    margin: 0px;
 }
 
 QScrollBar::handle:vertical {
@@ -178,6 +179,7 @@ QScrollBar::up-arrow:vertical, QScrollBar::down-arrow:vertical, QScrollBar::add-
 QScrollBar:horizontal {
     background-color: rgb(58,57,58); /* dark */
     height: 14px;
+    margin: 0px;
 }
 
 QScrollBar::handle:horizontal {