Improve terrain patterns list with filter options

The terrain patterns list is almost useless at the moment, unless your terrain set has a complete set of transitions, which is almost never the case with mixed sets.

Adding a set of filter options would make it a lot more useful. Some examples:

* Only show the dim (defined) or bright (undefined) patterns.
* Show patterns containing exactly N colours (or at most N colours, or at least N colours).
* Show patterns containing only colour N, or colours N and M, etc.
* Exclude patterns that are rotationally or reflectionally symmetric (possibly based on the tileset settings).
* Show patterns that contain a particular configuration of one colour (and the rest unspecified) – for example, "all corner tiles". (This might be a bit too complex though.)
* For mixed patterns, show only the 47 (or 48?) tiles used by a blob tileset.

Besides filtering, I'd like to see a number on each pattern that shows how many tiles have that pattern; and for isometric tilesets, I'd like to see the pattern rotated 45°, the same as you see when painting it.

Start by locating the terrain patterns list in the editor and reviewing how mixed terrain sets and isometric tilesets are currently displayed. Define which filter options, tile counts, symmetry handling, and rotated previews belong in scope; done means the agreed options work and the list presents the requested counts and previews.

Written by the indexing model from the issue text.