LibGUI+HackStudio: Make gutter a first class element

Gutter -- a space left of the text, before the ruler -- is not a part of
the ruler, nor should it be treated as such. This commit implements
gutter handling in LibGUI::TextEditor as part of mild cleaning up of the
gutter handling (breakpoint icons) in HackStudio's Editor.

This commit also enables separate theming of the gutter.

Userland/Libraries/LibGfx/SystemTheme.h

@@ -27,6 +27,8 @@ namespace Gfx {
     C(ButtonText)                  \
     C(DesktopBackground)           \
     C(FocusOutline)                \
+    C(Gutter)                      \
+    C(GutterBorder)                \
     C(HighlightWindowBorder1)      \
     C(HighlightWindowBorder2)      \
     C(HighlightWindowTitle)        \