Prepare for release 8.0.0

[jeremypw]

• Add to release section to metadata for v8.0.0
• Update screenshot paths
• Merge essential fixes
• • Jeremypw/plugins/fix libpeas2 #1566 Required for plugins to work with libpeas2
• • Fix global search - load children synchronously when expanding sidebar to path #1571 Required for global search to work reliably with asynchronous folder loading
• • Fix natural copy-paste in terminal - native behaviour when no selection/empty clipboard. #1588 Fix natural copy - pass on 'Ctrl C' when nothing to copy
• • Investigate possible crash from symbol pane found in dogfood testing branch - found not to apply to original repo
• • Reapply accidentally reverted Increase target size in plugin list #1567
• Merge any desired outstanding PRs if possible
• • Show more information in Vala symbol tooltip #1565 Also stops symbol pane blocking UI when parsing
• • Increase target size in plugin list #1567 Bitesize
• • Sort branch lists alphabetically #1576 Bitesize
• • Follow natural-copy-paste behaviour of pantheon-terminal #1578 Nice to have - fixes long outstanding request
• Finalize

[jeremypw]

To not hold this up anymore I'll leave a couple of terminal pane related PRs for the next (minor) release, which I will make soon.