Make main view react to context menu events

Before this commit, Dolphin's main view would not react to any context menu events. It only showed context menus based on hard-coded mouse or keyboard events i.e. mouse right-click and presses of the "Menu" key.

This commit removes those hard-coded reactions and instead makes it so the view shows a context menu whenever a QContextMenuEvent is received. Therefore, a context menu will now be opened when any platform- or system-specific context menu triggers are invoked e.g. the Shift+F10 keyboard shortcut, aside from the usual mouse right-click or "Menu" key press.

Aside from this, the only side-effect is a partial removal of an unrelated bug: Previously, the hover highlight on items was never cleared when the header column in details view mode was hovered. With this commit, the hover is now correctly cleared most of the time.

Test plan

Try to find unexpected or changed behaviour by using mouse movement and mouse buttons. Open context menus either by mouse or keyboard shortcut and see if anything is wrong. Shift+F10 should open context menus in the main view now. @teams/qa, we wouldn't want any unexpected behaviour changes to sneak in. I think I tested this somewhat thoroughly already though. ^_^ But I don't have a touch device so I couldn't test that.