BrowserPanel: Fix GUI update not running on JavaFX thread
BrowserPanel: Fix GUI update not running on JavaFX thread

loadPage(String) must run on the JavaFX thread.

The current implementation doesn’t ensure that this method runs on the
JavaFX thread.

Let’s update the method to ensure that this method runs on the JavaFX
thread.

damithc/addressbook-level4,se-edu/addressbook-level3,CS2103R-Eugene-Peh/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4,damithc/addressbook-level4