Updates to build on Ubuntu 18.04 LTS (…)

- 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

src/SeerTildeLogWidget.cpp

@@ -12,17 +12,29 @@ void SeerTildeLogWidget::processText (const QString& text) {
 
     QString str = text.mid(1); // Remove leading "~"
 
+#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
     if (str.front() == '"') { // Remove leading """
+#else
+    if (str.at(0) == '"') { // Remove leading """
+#endif
         str = str.mid(1);
     }
 
+#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
     if (str.back() == '"') { // Remove trailing """
+#else
+    if (str.at(str.size() - 1) == '"') { // Remove trailing """
+#endif
         str.chop(1);
     }
 
     str = Seer::filterEscapes(str);
 
+#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
     if (str.back() == '\n') { // Remove trailing "\n"
+#else
+    if (str.at(str.size() - 1) == '\n') { // Remove trailing "\n"
+#endif
         str.chop(1);
     }