EFA#141 Disable plugin download functionality

[cubitusgh2]

In the early days of efa (Year 2000 to 2010), the installation packages of programs had to be as small as possible as the download was performed over landlines with little transfer rates.

So the optional Elements of efa were put into "Plugins" which could be downloaded when needed AFTER the primay installation of efa.

Nowadays this is no longer valid. The distribution package of efa always contains all libraries and plugins.

So as a first attempt, the download feature for the plugins shall be disabled,
and the plugin menu should be removed (later).

[cubitusgh2]

See https://github.com/cubitusgh2/efa_ksc_replay/tree/MN_EFA_141_DISABLE_PLUGIN_DOWNLOAD

[cubitusgh2]

As of efa 2.5.1, the efa plugin infos are for information only in the efaAbout dialog.
The possibility to install plugins from efa.nmichael.de has been removed, as the
plugins are always included in the distribution of efa - there is simply no need to download plugins afterwards.

The plugin info in plugins.xml is not outdated yet, as the libraries/plugins need to be within the classpath
so that java can instantiate them. If the libraries cannot be found, some functions of efa may fail.
To be able to debug this (detect that the libraries can be instantiated), this code is still neccesary.