kivy: add "clear all gossip" button in ln gossip dialog

One usecase is perhaps to save space if using trampoline anyway...
more importantly, if using gossip, LNGossip is heavily filtering what messages we request and get,
and e.g. can missing new NodeAnnouncements, etc,
and this is a quick-and-dirty workaround to force a fresh start.

electrum/channel_db.py

@@ -51,6 +51,7 @@ if TYPE_CHECKING:
     from .network import Network
     from .lnchannel import Channel
     from .lnrouter import RouteEdge
+    from .simple_config import SimpleConfig
 
 
 FLAG_DISABLE   = 1 << 1
@@ -304,7 +305,7 @@ class ChannelDB(SqlDB):
     NUM_MAX_RECENT_PEERS = 20
 
     def __init__(self, network: 'Network'):
-        path = os.path.join(get_headers_dir(network.config), 'gossip_db')
+        path = self.get_file_path(network.config)
         super().__init__(network.asyncio_loop, path, commit_interval=100)
         self.lock = threading.RLock()
         self.num_nodes = 0
@@ -328,6 +329,10 @@ class ChannelDB(SqlDB):
         self.data_loaded = asyncio.Event()
         self.network = network # only for callback
 
+    @classmethod
+    def get_file_path(cls, config: 'SimpleConfig') -> str:
+        return os.path.join(get_headers_dir(config), 'gossip_db')
+
     def update_counts(self):
         self.num_nodes = len(self._nodes)
         self.num_channels = len(self._channels)