[Linux] Also put copied text into the primary selection buffer

[baconwaifu]

One of the nicer features of linux is that there's actually multiple clipboards: the "traditional" ctrl-c/ctrl-v clipboard, and the "primary selection" which contains what you last highlighted, and is pasted with middle-click where the mouse cursor is. QClipboard treats these as 'modes' via a parameter, so it's just an additional setText(text, Selection) call if clipboard.supportsSelection() returns True to populate it as well. (Given that highlighting text to copy it also puts it in the primary selection, I'm occasionally surprised when something that modifies the clipboard doesn't update the selection buffer as well)

It would allow quick transcription of multiple text bubbles without needing to touch the keyboard, by just highlighting text, then moving the mouse over to a text document and middle-clicking to paste it, and shouldn't interfere with normal operation or platforms that don't support the selection buffer.