Use output names instead of screen indices

Summary:
Screen index as an identificator can be pretty unreliable, since it can
change during the runtime (e.g. on login when kscreen applies primary
screen setting)

Use output name (e.g. HDMI-0) everywhere instead

ScreenMap: use raw ScreenSpace strings as mapping keys
ScreenSpace: introduce method next() that cycles through screen spaces
(monitors and whole desktop)
TabletAreaSelectionController: store ScreenSpace instead of index
X11Info: add helper functions to navigate through screens
TabletDaemon: remove outdated FIXME comments

Test Plan:
the usual see if there aren't any regressions in saving/restoring settings
inclduing Ctrl+Meta+1/2/F/M hotkeys that cycle through screens
initially old mapping settings are going to be lost
since map0/map1/etc. aren't valid screenspaces anymore

Reviewers: xuetianweng, fvogt

Differential Revision: https://phabricator.kde.org/D10274