Implementing tags

[flowgunso]

The tags feature have been mentionned in #40, #66, #216 and #46. I'm considering to PR this.

At least the following feature would be a great addition:

• List all tags
• List photos associated with a tag

But could also be possible:

• Search for tags (#46)
• Show tags of a photos
• List photos associated with several tags
• List tags related to a tag
• Adding, renaming, merging and deleting tags

Focusing on the first two features:

List all tags

Add a screen accessible by the side menu, list the tags sortable by name and count.

List photos associated with a tag

Add a screen when navigating to a tag that list photos associated with it.