Give text selection highlight a different color if the UI element is out of focus.

[billyswong]

Discussion stemmed from https://forums.linuxmint.com/viewtopic.php?p=2277794

Due to some design flaw of file chooser dialog of GTK, the insufficient distinction between selected text of a in-focus text box and selected text of a unfocused text box induce confusion to some users. The GTK team tried to auto refocus the dialog to the file name text box after clicking sidebar etc but sometimes this patch leaks and cause a lot of confusion. If we can make highlighted text of unfocused text box clearer that it is unfocused, there will be a lot less confusion.