Add add-on installation hooks

[abdnh]

A follow-up to #2518

This adds the addons_manager_will_install_addon and addons_manager_did_install_addon hooks. My main use case is to prevent update errors caused by the add-on holding a DB connection to some file in the add-on folder. Example usage:

def on_addon_manager_will_install_addon(manager, module):
    if module == manager.addonFromModule(__name__):
        db.close()


def on_addon_manager_did_install_addon(manager, module):
    if module == manager.addonFromModule(__name__):
        db.init()


gui_hooks.addon_manager_will_install_addon.append(
    on_addon_manager_will_install_addon
)

gui_hooks.addon_manager_did_install_addon.append(
    on_addon_manager_did_install_addon
)

Another common source of installation errors are open .pyd files coming from bundled dependencies. The AnkiHub add-on for example patches AddonManager.deleteAddon twice to work around similar errors. (I probably have a bunch of more add-ons suffering from this issue too).

dc066bb fixes a crash in .deleteAddon() when run from the update routine and OSError is thrown there.

[dae]

Thanks Abdo!