Allow plugins to disable other plugins

[edreamleo]

We don't want to change how Leo handles @enabled-plugins nodes. Any such changes would be inconvenient for existing users.

However, we want the vr2 plugin, if enabled and successfully loaded, to be able to disable the vr plugin. This should be possible regardless of whether vr2 is enabled in myLeoSettings.leo or the local .leo file.

The plugins manager already has the unloadOnePlugin method. This probably is not sufficient to completely suppress the vr plugin from the vr2 plugin. More likely, some more data will be needed in the plugins manager, and a new interface.

[edreamleo]

After more thought, I think this is a bad idea. It assumes that plugins can know the order in which Leo loads them, but that isn't so.

Furthermore, the vr and vr2 plugins load as expected in the following scenarios:

• leoSettings.leo enables vr (in an @enabled-plugins node).
• myLeoSettings.leo enables vr2.
• ekr.leo enables vr.
• test.leo enables neither plugin.
• test2.leo enables both plugins.

This works as expected with both PyQt4 and PyQt5:

• With PyQt5, the vr plugin in enabled in ekr.leo, test.leo and test2.leo.
• With PyQt4, both vr and vr2 are enabled in test2.leo.

Furthermore, both vr and vr2 can be active at the same time. They create separate vr panes!

For all these reasons, this issue is a bad idea.