Add static+const qualifiers

[mstoeckl]

The three commits in this PR add const (and in one case, file-static) qualifiers to global variables that were missing them.

• The list of atoms in xwayland/xwm.c
• The cursor data used by the default theme
• A number of "implementation function lists", which include structs of function pointers for wlroots backends, and for handling Wayland events and requests

Since xwayland/xwm.h is an internal header, and wlr/interfaces/wlr_tablet_tool.h and wlr/types/wlr_tablet_tool.h are unstable API, I presume changing types slightly is fine.

I found these variables by checking objdump -t libwlroots.so.7 | grep "\.data" for initialized global variables not linked as read only. I did not look at tinywl or any of the examples.

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Breaking change: wlr_tablet.impl is now const.

[emersion]

Thanks!