wxGUI/gui_core: fix import PostGIS DB raster

[tmszi]

Describe the bug
Import PostGIS DB raster via wxGUI Import raster data dialog doesn't work.

To Reproduce
Steps to reproduce the behavior:

1. Launch wxGUI
2. Export some raster map into GTiff format e.g. r.out.gdal input=elevation output=/tmp/elevation.tif and import with raster2pgsql -I -C -s 3358 /tmp/elevation.tif public.elevation | psql -d <YOUR_DATABASE> -U <YOUR_DATABASE_USER> -h <YOUR_DATABASE_HOST> -p <YOUR_DATABASE_PORT> to PostGIS DB
3. Set PostGIS DB connection via db.login module
4. From the Data catalog toolbar activate Import raster data tool
5. From the Import raster data dialog Source type choose Database RadioButton widget
6. From the Source input Format ComboBox widget choose PostGIS Raster driver
7. From the Name ComboBox widget choose your database name
8. List of raster layers ListCtrl widget is empty, no raster layers are loaded.

Expected behavior
Loading PostGIS DB raster layer and importing via wxGUI Import raster data dialog should be work.

Screenshots

Current behavior:

Expected behavior:

System description (please complete the following information):

• Operating System: all
• GRASS GIS version: all

[petrasovaa]

I can't review/test this in detail, so I just left couple notes/questions.

Thanks!