Add update/about menu

[shuvashish76]

Is your feature request related to a problem? Please describe.
Updating the plugin is an issue since KOReader has no inbuilt store feature
.

Describe the solution you'd like
Update option similar to Bookshelf plugin.

Describe alternatives you've considered
Manually download and update the plugin.

[smartscripts-nl]

On my Boox Go 10.3 Bookshelf immediately crashes when I try to call it. And the update functionality also results in an error: the update file wasn't downloaded. So not very encouraging and motivating for me to install the update-functionality in DX!