Tweaks to search edit

Some minor tweaks to the search edit.

* Make Ctrl+K (Cmd+K on mac) focus the search box and select the currently entered query (does not select the docset filter if one is available).

* Make escape anywhere in the main window work as pressing escape in the search edit.

Also:

Slight tweak to the search edit <-> treeview interaction. Now move actual focus when pressing down in the search box -- and fix the tab order so shift+tab moves back to searchedit.

Move the escape handling into the search edit instead of the line edit.