Set stock icon for speed, options and statistics buttons in qt

[buckmelanoma]

This PR sets stock system icons for the alt-speed, options and statistics buttons on the status bar in transmission-qt, replacing the poorly scaled PNG icons when possible. This essentially fixes #2174

[varlesh]

@buckmelanoma Please added for this PR another icon name for Open URL button. On Qt it's looking very ugly:

addEmblem(icon_open, QStringList() << QStringLiteral("emblem-web") << QStringLiteral("applications-internet")));
Emblems used for folders, but not for actiions icons on toolbar and overlay icons.
On all icon themes (Adwaita, Breeze, Faenza) available normal icon for that insert-link:
ui_.action_AddURL->setIcon(getStockIcon(QStringLiteral("insert-link"), QStyle::SP_DesktopIcon));

[buckmelanoma]

@varlesh I have a second PR in the works that fixes the menu and toolbar icons. For AddUrl I think the proper icon is document-open-remote.

[ckerr]

I agree with the problem and most of this PR, but have a soft spot for that turtle icon. It's been with Transmission from the very beginning on all platforms so I'd rather see it get replaced with a well-scaled / high-contrast friendly version of the same turtle.

Could you update this PR to leave the existing turtle (or, if you're a graphics artist, to update it with a better revision of it)?

[buckmelanoma]

I tried an upscale of the turtle in Krita but it didn't turn out too well. I've updated PR to revert system override of the classic turtle.

[ckerr]

Thank you for trying!

Yeah, we will probably need to redo the image from scratch in svg.