Improve python debugger client. (#2441)

Replace DisplayData to DebuggerAction. The new class has only four type
options. Furthermore several functions returning with DisplayData is
changed to return with string.

JerryScript-DCO-1.0-Signed-off-by: Zoltan Herczeg zherczeg.u-szeged@partner.samsung.com

tests/debugger/do_delete.expected

@@ -7,7 +7,7 @@ Stopped at tests/debugger/do_delete.js:17
 Breakpoint 1 at tests/debugger/do_delete.js:17
 (jerry-debugger) b do_delete.js:21
 No breakpoint found, do you want to add a pending breakpoint? (y or [n])
-Pending breakpoint at do_delete.js:21
+Pending breakpoint 2 at do_delete.js:21
 (jerry-debugger) b do_delete.js:19
 Breakpoint 3 at tests/debugger/do_delete.js:19
 (jerry-debugger) b do_delete.js:18
@@ -20,7 +20,9 @@ Breakpoint 4 at tests/debugger/do_delete.js:18
 === Pending breakpoints ===
  2: do_delete.js:21 (pending)
 (jerry-debugger) delete 2
+Pending breakpoint 2 deleted
 (jerry-debugger) delete 3
+Breakpoint 3 deleted
 (jerry-debugger) list
 === Active breakpoints  ===
  1: tests/debugger/do_delete.js:17