Library sidebar: also activate items on PageUp/Down events

[ronso0]

Activate sidebar items also on PageUp / PageDown / Home / End events = update the table view
Affects keyboard and [Library],Scroll.. commands.

Previously those keys would move the sidebar selection as expected but the new item wasn't actually activated.
The table view didn't change, may even stay empty, which does cause panic in case you scroll up to Tracks ; )

[ronso0]

for the sake of completeness I'll also add Home/Pos1 and End

[ronso0]

the second commit is supposed to facilitate the navigation a bit:
Left now selects & activates the parent item. I'm used to it from some other app's tree view (Edit: Thunar and Thunderbird) and I find this very handy, e.g. when you're somewhere in the middle of a long crates list and want to jump to another root item (using PageUp/Down and Up/Down afterwards is cumbersome)

[ronso0]

rebased on 2.3 since the PageUp issue is a bug actually.
Anyone around who cares to review this small fix?

[ronso0]

the second commit is supposed to facilitate the navigation a bit:
Left now selects & activates the parent item. I'm used to it from some other app's tree view (Edit: Thunar and Thunderbird) and I find this very handy, e.g. when you're somewhere in the middle of a long crates list and want to jump to another root item (using PageUp/Down and Up/Down afterwards is cumbersome)

I removed that commit. I does not yet work correctly for 2+ child levels. Once I figured that out I'll target it at main.
see #4253

So this simple fix is ready for merge @mixxxdj/developers

[daschuer]

LGTM, thank you.