Add UI and dummy rendering

[ArchibaldBienetre]

Implements https://github.com/SergelsOrg/csv2tex/issues/8

Draft status:

• [x] wait for merge of #43
• [x] rebase
• [x] implement JavaFX UI + TestFX tests
• [x] wire up UI with existing rendering code from #43

[ArchibaldBienetre]

Merge conflicts because of #46 - rebase will take me a bit of time