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.

Libraries/LibGUI/TreeView.cpp

@@ -377,7 +377,8 @@ void TreeView::did_update_model(unsigned flags)
 void TreeView::did_update_selection()
 {
     AbstractView::did_update_selection();
-    ASSERT(model());
+    if (!model())
+        return;
     auto index = selection().first();
     if (!index.is_valid())
         return;