plugin: make PluginDependencyGraphBuilder pass new "provides" tests

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: extract common method from PluginDependencyGraphBuilder

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: move more responsibilities to PluginActivationService

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: add new class PluginConflictResolver

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

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