Facilitate filtering by platform version in Vaadin Directory

[javier-godoy]

When Vaadin Directory is loaded, the filters read "Vaadin: platform, polymer: 3 and GWT: 2" but actually no filter is applied. Thus, in order to filter by Vaadin platform, one needs to select Vaadin 8 and then select platform again.

Now a "minor version" combobox appears, where you can select the Vaadin 10+ major version.