LocaleManager: set system locale and keyboard via dbus

[danirabbit]

As recommended in #147 (comment)

[ryonakano]

Also, maybe we can update the data/locale.policy.in file?

• L18 can be removed since we no longer use localectl with pkexec command
• org.freedesktop.locale1.set-locale in L19 can be removed since we no longer check for permission when set_system_button is clicked

https://github.com/elementary/switchboard-plug-locale/blob/425a6642d3361dd82a6625e3ef45567e07a53fa0/data/locale.policy.in#L18-L19

[danirabbit]

@ryonakano thanks for the thorough review! Addressed those feedback points :)

[ryonakano]

Just one small nitpicking, otherwise looks good and works as expected.

[danirabbit]

@ryonakano thanks for catching that!

[ryonakano]

Code looks good and works as expected. Thank you for your work!

[danirabbit]

Thanks for your thorough review!