Various UI and Wikipedia context applet improvements

High-DPI inline playlist controls, stop-after marker, fix inline progress slider knob

Don't show play mode controls when in dynamic mode, they have no effect there

BUG: 287055

Show a hint that context view applets can be resized in edit mode

Cleanup commented-out code from playlist UI & unused files from earlier Wikipedia applet versions

Update Wikipedia applet to QtControls2, show only relevant items in right-click menu.

BUG: 323941

Ignore non-Wikipedia links in Wikipedia applet, use the monobook skin also when opening links inside the applet. Fix opening of redirections to other Wikipedia pages. Disable broken scrollbar, replace with a slider. Fix searching for composer and album. Display missing metadata warnings from backend in Wikipedia applet ui.