Scroll through chars in user definable character sets with the mouse wheel
- allow H and V mirroring for currently active character
- allow 90 degrees rotation for currently selected char
How do we deal with characters that do not have a counterpart?
Example: Character "A"
- can do H mirror (basically no change)
- can not rotate by 90 degrees
- can not rotate by 180 degrees
Example: Character: "O"
- can do H mirror (no change)
- can do V mirror (no change)
- can not rotate 90 degrees
- can do 180 degrees (but would that be possible when 90 degrees was already not possible?)
While a pattern matching approach (saw the code, absolutely awesome) is super smart, it would not cover other situations that might be useful, too, e.g. the different horizontal lines:

How about this suggestion:
- define character sets (can be anything from 2 to x characters)
- scroll wheel to cycle through them
Or combine both ideas:
- Is character part of a set? ** yes: cycle to next character from that set ** no: try 90 degree rotation *** if that fails, try 180 degrees *** if that fails, try 270 degrees *** if that fails, no change
That way you deal with all characters and have "smart" sets.
So I'm putting off the selection set thing but I'd like to have 'r' rotate the currently selected character, just like it does for a brush.
But when rotating, how should the character selection pane react? Should it re-select the current char to whatever was chosen by rotation? Or should it keep the current selection but rotate only the char so that the rotated character shows as rotated on top of the painting canvas? The latter is sort of easier to implement. But would this feel like a bug that the selection doesn't change?
ESC would reset rotation & flips to defaults. Ditto for choosing another char from the char selector.
There's a bit of redux plumbing to be done to implement this well. It's not enough to just look up the rotated char code and set the selected char, and then on the next 'r' hit look up the next rotated char code and set the selected char.. Because not all rotations/flips are defined, so there needs to be a separate redux value that keeps track of rotation angle + flips (like there is for brushes).
The 'r' shortcut is done and it is done so that it feels awesome!
Putting off the rest of this bug to 0.5 or later. The UI part would need to be designed in more detail. Also it feels a bit complex so maybe implement once someone feels super strongly about the need for this.