wxGUI: Add new icons to data catalog for actions avaliable in context menu

[lindakarlovska]

Certain actions that are currently available (and will be available in the future) in the context menus of data catalog need to be exposed as toolbar icons. This is a suggested list of actions, to be edited based on discussion and feature availability:

• add new database (already created)
• create new mapset (now created)

Other:

• download sample location
• create new location
• import into current mapset

[petrasovaa]

There is already an icon mapset-add.png in grass you can use.
To create location-add icon, combine QGIS icons mIconOws.svg and mActionAdd.svg
To create download location icon, use the import symbol (like in grass layer-import.png) - the svg of that can be found in qgis mAddToProject.svg.

[petrasovaa]

Although this works, this is not what we discussed. In tree.py you need to create a function CreateMapset that would be called from OnCreateMapset in tree.py, catalog.py and frame.py, which are all event handlers just differing in how they get the parameters for CreateMapset, which does the actual work of creating the mapset and adding it to the tree.

[petrasovaa]

@lindakladivova I edited it (I suggest to look at the changes) and merged it to finish this for now. Create a new PR for the location icons.