improve/fix painting of drag-and-drop insert indicator lines on HiDPI screens

[DevCharly]

The drag-and-drop insert indicator lines for TreeView, OutlineView and Palette were painted using six g.drawLine() statements, which work fine for 100% screen scaling:

But not so good on 150% screen scaling (you can see the six single lines):

This PR changes painting to use a single filled shape and enables antialiasing. At 100%:

At 150%:

This PR also sets the painting color of the D&D line for the palette (was not set before before):

Also fixed some compiler warnings (unused imports, missing Override, ...)

[eirikbakke]

Ah, beautiful. The old rendering has annoyed me on HiDPI screens for some years...

I'm doing a new NetBeans build now and will let you know when I have tested this and the other PR.

[eirikbakke]

Tested and works in my private IDE build and NetBeans Platform application! Thank you!

[matthiasblaesing]

Only tested with normal resolution and there it still looks like it should. The approach looks sane, lets get this in.

[eirikbakke]

My test was done on 150% DPI scaling on Windows.