Revamp Wayland application identification; align Wayland and X11 models

Summary:
This adds support for app ids which are absolute paths to .desktop
files to WaylandTasksModel, matching the heuristic in XWindowsTasksModel.

Behavior between the two models is made more similar in various ways,
such as matching codepaths for generating the ids inserted into the
KActivities database in requestNewInstance/requestOpenUrls (also the
X11 model was missing the notifyAccessed calls) and producing launcher
URLs.

Icons are still left to KWin however, instead of reading them from
KService (which is what KWin does in any case on Wayland) or from the
.desktop file. The latter might make sense, but to avoid duplicated
code would then best be done in KWin.

Reviewers: #plasma, graesslin, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3292