Set specific edge cursor shape when resizing

Summary:
Instead of seeing the cursor <--> on the left edge you now see an icon
that looks like |<-  .

This brings kwin decorations in line with GTK CSD icons.

In theory this is also useful to tell which window will resize in the
case of side-by-side windows (regardless of whether borders are on or
not). In practice with the adwaita icon theme I tested with it's not
very intuitive to realise which is which till you learn the icon.

Change is more involved than it should be as Qt::CursorShape doesn't
have these entries, and I don't want to shadow that enum internally or
have
to change kwin effect code.

Specifics depend on cursor icon theme if they are not present it will
fallback to the <--> icon. (Breeze does not have them currently)

Test Plan:
Resized some windows (on X and on Wayland)
Correct icon appeared on Adwaita
Existing icon appeared on Breeze

Reviewers: #plasma

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13...