Allow entries from the left list to be middle-mouse button'ed

[Miepee]

Description

This PR makes it so that entries from the left list can be middle-mouse button'ed, which opens them in another tab.
Fixes #981

Caveats

• If you attempt to MMB the headers, they won't get highlighted, which may look a little weird
• MMB'ing results in the treeview centering the item that was clicked, which may look a little weird.

[github-actions]

Download the artifacts for this pull request here:

GUI:

• GUI-windows-latest-isBundled-false-isSingleFile-false
• GUI-windows-latest-isBundled-false-isSingleFile-true

CLI:

• CLI-macOS-latest-isBundled-false
• CLI-ubuntu-latest-isBundled-false
• CLI-windows-latest-isBundled-false

[VladiStep]

Besides the caveats you've described, there are more issues:

1. The MouseDown handler is MainTree_OnMouseDown(), but it's assigned to every child of TreeViewItem.
2. It doesn't check the UndertaleRoom layers order that you are opening.
3. It doesn't open UndertaleString items.

[Miepee]

The MouseDown handler is MainTree_OnMouseDown(), but it's assigned to every child of TreeViewItem.

It is supposed to be assigned to the TreeViewItem children. The name is completely wrong, because it was from when I was still figuring things out which I later forgot to update xP

[Miepee]

Closed in favour of #988