wxGUI: get rid of gradient in map display notebook tabs

[petrasovaa]

Applies to single window layout only.
Based on https://github.com/wxWidgets/Phoenix/blob/master/wx/lib/agw/aui/tabart.py

[lindakladivova]

@petrasovaa I think that it is definitely better without gradient. :-) Only one thing which is probably not much related to this PR - I have tested it with wx version of 4.1.1.

For comparison:
With gradient.:

Without gradient:

I cannot help myself but I do not like the sharp black notebook page's border. I have not searched for when they did the change but in my previous version 4.0.1, it looked much better.

[petrasovaa]

I cannot help myself but I do not like the sharp black notebook page's border. I have not searched for when they did the change but in my previous version 4.0.1, it looked much better.

It doesn't look like that for me. I think the color is probably defined here:

self._border_colour = wx.SystemSettings.GetColour(wx.SYS_COLOUR_BTNSHADOW)

So it would depend on your theme. Maybe we can merge this and tweak it later, I don't really know how this will look like on mac and windows.