Updates to build on Ubuntu 18.04 LTS (…)

[ernstki]

• replace add_compile_definitions (added in CMake ≥3.12) with add_definitions in CMakeLists.txt

  • hat tip to: https://stackoverflow.com/a/61250302/785213

• add #ifdef's to support Qt 5.9.5 for Ubuntu 18.04 by replacing QString .front() and .back() methods with .at(0) and .at(s.size() - 1)

  • hat tip to: https://github.com/pawelsalawa/sqlitestudio/commit/90ca398

• add #ifdef's to replace QFontMetrics .horizontalAdvance() method with .width() to support Qt 5.9.5 on Ubuntu 18.04

  • hat tip to: https://github.com/Sigil-Ebook/Sigil/commit/a32f890

• closes #107

[ernstki]

I also de-dented sections of the README that appeared to be accidentally indented (they were showing up as preformatted text when rendered on GitHub).

[epasveer]

Thanks for your work!