STL containers can be expanded only at top level

**Describe the bug**
STL containers can be expanded only at a top level

**To Reproduce**

- compile and open in gdbgui
- run program until `return 0;`
- take a look at local variables

```cpp
#include
#include

int main(int argc, char *argv[]){
std::vector v(7, 0.3);

std::vector> vv(2, std::vector(3, 0.4));

std::map> m;
m[1] = std::vector(2, 0.5);
m[2] = std::vector(3, 0.7);

return 0;
}
```

**Expected behavior**
Nested containers must be expandable.

**Screenshots**

Some `{...}` things cannot be expanded


GDB CLI prints it well


System info:

* OS: Linux Mint 18.3
* gdbgui version (`gdbgui -v`): 0.13.1.1
* gdb version (`gdb -v`): GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1
* browser [e.g. chrome, safari]: firefox 64.0
* python packages (`pip freeze`):
```
Click==7.0
Flask==0.12.4
Flask-Compress==1.4.0
Flask-SocketIO==2.9.6
gdbgui==0.13.1.1
gevent==1.4.0
greenlet==0.4.15
itsdangerous==1.1.0
Jinja2==2.10
MarkupSafe==1.1.0
pygdbmi==0.9.0.0
Pygments==2.3.1
python-engineio==3.2.3
python-socketio==3.1.1
six==1.12.0
Werkzeug==0.14.1
```