Themable character choices

Right now it seems that most of the characters that Cursive outputs are chosen in string literals in the source code for the components that implement them. It would be nice for themes to be able to choose how to draw certain widgets. In Unicode 13, a new block (called Symbols for Legacy Computing) has been added containing characters from old home computers which are specifically meant for TUIs. By allowing themes to make use of these, it would be possible to create themes that look like this for Cursive.


The above TUI is made entirely using characters from the Apple II's MouseText character set (which was incorporated into this new block). Of course, since font support for these characters can't be relied upon in all terminals, having this be a feature of themes would be useful, since it allows the program to fall back to more commonly available box-drawing characters.