plugin: move widgets from gui/xxx to plugin/gui

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

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: use new dialog to show messages about deactivation

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

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: #2706 fixed: smarter loading and unloading plugins

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