plugin: added stub of warning when deactivating more than one plugin

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: remove dependency on PluginActivationService in PluginModel

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: remove PluginEntry class

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: check pluging dependencies in widget using widget's datai

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: remove PluginDependencyHandler dependency from PluginModel

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
gui: fix all menu items added by plugins

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>