LibGUI: Add Ctrl-H to insert mode

In Vim, Ctrl-H is effectively equivalent to backspace in insert mode, as
it deletes the previous character.

This commit implements method delete_previous_char() to TextEditor.
delete_char() already exists in EditingEngine, but it deletes the
next character rather than the previous. delete_previous_char() is then
called from within VimEditingEngine.

Userland/Libraries/LibGUI/TextEditor.h

@@ -136,6 +136,7 @@ public:
     void do_delete();
     void delete_current_line();
     void delete_previous_word();
+    void delete_previous_char();
     void select_all();
     virtual void undo();
     virtual void redo();