Add support for database migrations.

README.md

@@ -55,6 +55,8 @@ $ brew cask install qt-creator
 $ brew install qt
 ```
 
+[Peewee Migrate](https://github.com/klen/peewee_migrate) is used to manage database migrations. Add them to the `vorta.migrations` package.
+
 To build a binary package:
 ```
 $ pyinstaller --clean --noconfirm vorta.spec 

setup.cfg

@@ -41,6 +41,7 @@ install_requires =
   python-dateutil
   keyring
   borgbackup
+  peewee_migrate
 
 [options.extras_require]
 tests =

src/vorta/__main__.py

@@ -3,6 +3,7 @@
 import peewee
 
 import vorta.models
+import vorta.migrations
 from vorta.application import VortaApp
 from vorta.config import SETTINGS_DIR
 
@@ -15,6 +16,10 @@
 sqlite_db = peewee.SqliteDatabase(os.path.join(SETTINGS_DIR, 'settings.db'))
 vorta.models.init_db(sqlite_db)
 
+# Run migrations
+from peewee_migrate.cli import get_router
+router = get_router(os.path.dirname(vorta.migrations.__file__), sqlite_db, True)
+router.run()
 
 app = VortaApp(sys.argv)
 sys.exit(app.exec_())

src/vorta/migrations/001_add-snapshot-size.py

@@ -0,0 +1,38 @@
+"""Peewee migrations -- 001_add-snapshot-size.py.
+
+Some examples (model - class or model name)::
+
+    > Model = migrator.orm['model_name']            # Return model in current state by name
+
+    > migrator.sql(sql)                             # Run custom SQL
+    > migrator.python(func, *args, **kwargs)        # Run python code
+    > migrator.create_model(Model)                  # Create a model (could be used as decorator)
+    > migrator.remove_model(model, cascade=True)    # Remove a model
+    > migrator.add_fields(model, **fields)          # Add fields to a model
+    > migrator.change_fields(model, **fields)       # Change fields
+    > migrator.remove_fields(model, *field_names, cascade=True)
+    > migrator.rename_field(model, old_field_name, new_field_name)
+    > migrator.rename_table(model, new_table_name)
+    > migrator.add_index(model, *col_names, unique=False)
+    > migrator.drop_index(model, *col_names)
+    > migrator.add_not_null(model, *field_names)
+    > migrator.drop_not_null(model, *field_names)
+    > migrator.add_default(model, field_name, default)
+
+"""
+
+import peewee as pw
+from vorta.models import *
+
+SQL = pw.SQL
+
+
+def migrate(migrator, database, fake=False, **kwargs):
+    migrator.add_fields(SnapshotModel, duration=pw.FloatField(null=True))
+    migrator.add_fields(SnapshotModel, size=pw.IntegerField(null=True))
+
+
+
+def rollback(migrator, database, fake=False, **kwargs):
+    """Write your rollback migrations here."""
+