Ability to define advanced divider properties

[paulh-rnd]

A bit of an issue log tidy up, and I came across this, I thought I'd break it out into its own issue for tracking:

I'd like to be able to define where I want the dividers. Right now it just divides into equal parts. In my case I also want one side open but dividers in the other side so being able to do half length dividers would be awesome (this is where things get hard).

Originally posted by @mshorter in https://github.com/paulh-rnd/TabbedBoxMaker/issues/12#issuecomment-316469527

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I like this idea, and I've often thought about how dividers could be done better: currently it's tedious to move them around (means moving key holes on multiple box faces, by exactly the right amount), and doing more complex layouts such as spanning. In fact it's a lot like building an old-style HTML table: define your rows and columns then define which cells span multiple rows or columns.

Unfortunately this is all a pipe dream at the moment while the code is so complex. I need to do some serious refactoring and Pythonic-ification, as it were.

[paulh-rnd]

Thoughts for divider features:

• Specify divider spacing as a string of ratios, eg. "2,3,5" would separate dividers with spacing in the ratio 2:3:5. What do we do with excess dividers (eg. 3 or more dividers selected instead of 2 in teh above example)
• Specify divider spacing in absolute length, eg. 20, 30, 50 (in the selected units). Same question on excess dividers, but also out of range values.
• Mixture of absolute and a wildcard to fill remaining width
• Possibly colour all the tab keyholes for each divider in a different colour to make grouping and moving easier? Can't really group them together because usually you want to move the holes with the box side.
• Spanning divider "gaps". This is complicated: need to add tabs to shorter dividers and key them into the face of other dividers, not just box sides. Probably not going to happen... :(

Feel free to add more suggestions!