plugin: reformatted PluginModel

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: remove unused ExtraRoles enum values

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: changed member names of PluginModel

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: update implementation of PluginModel

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

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: reduced includes for PluginProxyModel

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: reduced dependencies of PluginProxyModel

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: extract variables in PluginProxyModel::subSortLessThan

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