pinwidget: allow copying to clipboard and saving to file

[zhangfuwen]

add a context menu in pinwidget for copying to clipboard and saving to file

[mmahmoudian]

This addresses #1007 and consequently part of #954

[borgmanJeremy]

@mmahmoudian @veracioux what do you think of the way the menu is implemented. I wonder if instead we should show "purple bubbles" around the pin like the main menu.

@zhangfuwen Thank you for working on this!! It's a highly requested feature.

[veracioux]

@borgmanJeremy I agree that we should show purple bubbles, but the best, most consistent and maintainable way to do that is by adding a context menu item that turns the pin widget into a windowed capture GUI.

But for now, I like the context menu implemented in this PR.

[Alaskra]

When I try to save the pinned picture, the pinned picture covers the file manager and I can't move it away.

[Alaskra]

I think we should switch off the Qt::WindowStaysOnTopHint option, and then switch on after save.