Bump mvvmfx-validation from 1.7.0 to 1.8.0 (#4749)

Bumps [mvvmfx-validation](https://github.com/sialcasa/mvvmFX) from 1.7.0 to 1.8.0.
- [Release notes](https://github.com/sialcasa/mvvmFX/releases)
- [Commits](https://github.com/sialcasa/mvvmFX/commits)

Signed-off-by: dependabot[bot] <support@dependabot.com>

build.gradle

@@ -130,7 +130,7 @@ dependencies {
 
     // JavaFX stuff
     compile 'de.jensd:fontawesomefx-materialdesignfont:1.7.22-4'
-    compile 'de.saxsys:mvvmfx-validation:1.7.0'
+    compile 'de.saxsys:mvvmfx-validation:1.8.0'
     compile 'de.saxsys:mvvmfx:1.7.0'
     compile 'org.fxmisc.easybind:easybind:1.0.3'
     compile 'org.fxmisc.flowless:flowless:0.6.1'