allow changing an output's modelist at runtime

This should fix running Plasma in a windowed virtual machine, when the window is
resized, the mode list changes, and libksreen can't currently handle that.

Summary:
* make Output::modes() non CONSTANT, add modesChanged() signal
* compare the mode lists and set the new one
* queue an outputChanged signal when applied
* autotest for modelist changes
* update the mode list on RRNotify events

BUG:356864

Test Plan:
* for library part, autotests are added
* for xrandr backends, we can't sensibly autotest this :(

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: graesslin, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3117