Support int64 and uint64 in value_from_variant

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add doc Configuration and session storage

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Don't warn if old config value not set

Don't issue critical warning if old configuration value
has not been specified for Preferences dialog.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add doc Initialization and Preferences Forms

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>