LibGUI+HackStudio: Fix cursor appearance and crash while debugging

HackStudio uses a TreeView to display the list of current variables
while debugging, and when the program completes, it sets that view's
model to a null model. This would trip an assertion if the TreeView
had something selected at the time, so this patch lessens the
assertion into a simple null check.

Additionally, the cursor would look laggy when moving about the
editor because the code was waiting for a window repaint to update
the cursor's look when it makes more sense to update the cursor
when it actually moves. This change also requires the base
GUI::TextEditor to expose a getter to tell if its currently in a drag
selection.

Finally, requesting a context menu in the line ruler on the side of
the editor would also place/remove breakpoints, which was counter
intuitive, so this requires a left click to modify breakpoint placement.

DevTools/HackStudio/Editor.h

@@ -77,7 +77,6 @@ private:
     bool m_hovering_editor { false };
     bool m_hovering_link { false };
     bool m_holding_ctrl { false };
-    bool m_hovering_lines_ruler { false };
 
     Vector<size_t> m_breakpoint_lines;
     Optional<size_t> m_execution_position;