Improve clarity of current terrain painting mode when editing a tileset

Nobody has claimed this yet.

When painting terrain markings, sometimes the current selection changes unexpectedly.

Sometimes switching to the patterns view switches to the first solid tile; sometimes it doesn't. This should be more consistent. I think, generally, switching views probably shouldn't change the selected mode.

Clicking on a colour or pattern doesn't select it if it's already highlighted, which might sound fine on the surface; but being highlighted doesn't actually equate with being selected. The pattern and terrain colours list actually separately track the current selection, but for painting, only one of the two can ever be active. If you select a particular colour, then switch to the patterns view and select a particular pattern, then switch back to the terrains view and click the same colour, you're still painting the pattern, not the colour. If you click on something, it should be the active mode. I'm not sure whether this means that selecting a pattern should deselect the active colour (and vice versa), or if it means that clicking on something already selected needs to be handled.

It might also be nice to add a little preview in the corner that shows what's currently selected. This isn't that big of a deal though, since you can see by moving the mouse over the main tiles view.

Start in the tileset editor's terrain-painting controls and reproduce the reported sequence: switch between patterns and terrain views, then click already-highlighted colors or patterns. Done means view changes and clicks consistently activate the intended painting mode; clarify whether the optional selection preview is part of the scope.

Written by the indexing model from the issue text.