wxAuiNotebook - tabartgtk - gapbox not drawn, if active tab invisible

[jens]

If the active tab is not visible tabartgtk does not draw the gapbox.
The attached patch fixes this by drawing an gtk-box for inactive tabs.
For active tabs we draw a box without border, because some themes have transparent gaps and a line would be visible at the bottom of the tab (in the gap).
I also increased the height of the gapbox, because on some themes the bottom (or top, depending on alignment) part of the box is visible otherwise (should be hidden by the tab-content).

[jens]

(gap-)box when active tab is invisible

[VZ]

(In [72627]) Fix drawing of wxAuiNotebook with GTK theme when active tab is invisible.

Draw a box using gtk_paint_box() for the tabs, without border for the active
one to avoid an extra line across the gap, and with the border for the others.

Closes #14728.